I was wondering... How many population there is in equestria? Knowing that capital and ponyville was empty when they came back, I'm assuming that most of the equestrian population has been sent to earth. Does anyone know if there's specific number about it? I want to be specific about background and I don't think it's 'just' thousands...

I've always imagined Equestria to be fairly sparsely populated. But I think a better indicator for how many ponies could have been sent to Earth is not a guess on how many ponies lived on that world, but on how many people could have been born on May 1st. That number is no more than 400 000 worldwide.

I like your approach, but I think it's more than that. May 1st isn't the only day all people change into ponies. If we consider the fact that most of them would start to change in the following weeks or even months. I'm guessing over a million is possible.

How many ponies?
"Yes."
But that isn't a number.
"Did I stutter?"

7410925
Is that based on primary canon or just side stories? Because I don't recall any in the original story changing after May 1st. (characters from side stories where that happened were mentioned in the original story, but in very little detail and that aspect of those side stories wasn't mentioned to my recollection.)

It's based on the original one. Chapter 30 (ponies and ponies and ponies and...)

Mac gets up and goes over to hug the mares. Sunshine Clouds giggles. “Evan? Evan Smith? Haven’t talked to you since high school dude! But damn, it’s great to see a friendly face. Or, well, a pony version of an old friendly face! How long have you all been ponies?”

AJ taps her chin. “It all started May 1st. How about you?”

Lemon Hearts nods. “It was the first week of May for us.”

“I didn’t start to turn into a pony until May 11th!” One of the other new ponies pipes up.

and like I wrote here, rainbow dash was vanished to human world at 23:30. I don't think discord can empty the whole nation in 30 minutes.

7410925

The total population of Equestria is hard to calculate with any degree of confidence, considering any estimation involves a lot of unknowns that need to be extrapolated one way or the other – I've seen estimates ranging from a few hundred thousands to a few hundred millions. There's just not enough hard numbers in canon material to say, so this is mostly left to any author's headcanon. Discussion of the subject can be found on this thread.

To my knowledge we don't have a total population number outright stated in the Five Score universe (I thought I'd read one somewhere, but can't remember where... Please chime in if you do!). What we do have, however, are estimates of how many ponies could be on Earth, at least for the first months or years following May 1^st, 2020. This can give us a rough estimate of the percentage of ponies-cursed-to-be-human being born compared to the number of total births, and thus how many ponies would be present in various parts of the world, and it can also give us at least a general order of magnitude for the base Equestrian pony population – though again, only very rough estimates... Even if we consider that the rate of Discord banishing ponies to Earth decays exponentially during his 25 years of activity (which is the simplest scenario), the data we can gather is not precise enough to calculate a decay constant and thus estimate the starting population number.

Firstly, we don't have definitive numbers in the original Five Score story, set in May 2020. It's said that there could be 'hundreds, thousands of ponies' on Earth (Chp.28), and later the number of ponies reaching the farm and crossing the portal varies from 'a few thousand' (Chp.31) to 'a thousand' (Chp.32) to 'thousands' (Chp.34) to (somehow) 'tens of thousands' (Chp.35), concluding at 'over a thousand' (Chp.36). In any case, it's suggested that a substantial number of ponies are still present on Earth when the portal closes.

Turning now to sidefics, I've found an estimate of the number of ponies on Earth in Dust on the Wind. About two years after May 2020, when a character is asked about the number of ponies, she answers that she's 'aware of nearly quarter million ponies worldwide' (sic; Chp.46). There's of course the possibility that the character is underestimating that number, but if so it's hard to say by how much. What is certain is that this number is quite low, compared to the approximate total number of births worldwide for the May 1995 to May 1997 interval – over 264 millions (source). That'd give us a 'pony prevalence percentage' (or PPP) of about 0.09% for the corresponding time period. Using this number, we could then expect approximately 7,500 25-year-old US citizens would turn into ponies during the first two years after May 2020. This could be consistent with the lowest estimates from the original story. As for the pre-curse pony population in Equestria, we can thus say that it is a least greater than 250,000, and as this represents the number for only the first two of the 25 years, the real number could easily be double that one if not more – it really depends on how quickly Discord managed to curse ponies.

I've also found another rough estimate of pony numbers we could use in Five Score: A Velvet Nightfall, where a character estimates the number of ponies in the UK, on the basis of 'the percentages as (they) have it' in the US, as 'maybe a hundred or so total' (Chp.48); this estimate is given during the first six months following May 2020. From there, we would then have a PPP of roughly 0.03%, which, however, would translate to only about 530 US ponies during that timeframe, which we know isn't right. In this case, the characters are clearly underestimating the real number of ponies.

For my part, I think I've used a PPP of 0.1% to estimate the number of Equestrians that would be found where my stories take place.

Well, someone did the math!

I was just going to chime in to say that in the original version of the story we had this that could theoretically account for anyone not on Earth:

"Gilda found a set of stone gryphons and ponies in one of the halls. Turns out they were all alive, they were members of the resistance that Discord turned to stone in the weeks after his coup d'etat on Celestia."

But that's only present in the sfw version now. The nsfw online version was edited for this as well as the book I'm sure.

It could still be true and just not mentioned. There is at least one short story, "All That's Left Behind" starring Roseluck, featuring ponies in hiding in Equestria but I'm fairly sure they never interact with this lost group. Point is though, we could reason not the entire population may have been sent to Earth. As grim as that may be.