Any neatly rounded numbers have to be taken with scepticism especially with an open air venue. 20k was probably at the very top of what was then possible - I'm unclear though what the entrance criteria was - were premies dragging in friends and families or was it strictly premies only ?
I was very detached by 79 so have no personal insights but I recall various forum posts saying what a disaster Kissamee was so it might be fair to put Kissamee as the 20k highwater mark for the global (ex India) count, allowing 1975 to 1979 as a period of stable numbers. There was though perhaps a notable amount of churn - disaffected replaced with a similiar number of recruits but with premiedom becoming ever more US resident and the rest of the world numbers (ex India) falling as youth culture transformed from hippie to punk and beyond.
Interesting that hardcore US punk didn't really take off till the early 1980s and produced a cynical and militant youth culture that was never going to be amenable to Gurus. That culture change was coincidental with ashram closures and Prem's cult was adrift in a changing world becoming terminally white, middle class and aging.
Youth culture change in Europe - certainly the UK - was a couple of years ahead of the US and probably accounts for early decline of premiedom. I've argued in the past that post 1975 Rawatism should be seen as US cult, the US being the dominant culture in which it formed and in which it has been sustained.
It's a neat visual to think of Prem in 1971 on the same stage as Fairport Convention and Gong, but an impossibility that in 1976 he could share an event with the Sex Pistola and The Clash, let alone in 1981 with Henry Rollin's era Black Flag.
Prem's cultism parasitised youth but the youth changed and Prem became fat(ter), old and irrelevant Peace became 'rage', and the kids were no longer buying what Prem had to sell.
OK that was a lot longer than I meant