750,000 in total sounds about right. There are about 200,000 Indian students studying in US alone.
So add Europe, Canada, Australia etc and then rest of world, I would say it would be around that figure.
It is of course total number studying (say 4 year program) rather than number leaving each year to study which would some fraction of that.
Most would stay....get residency and then maybe citizenship depending on their career. Then they get work experience and career experience and remit some earnings back to India too.
This is overall what happens, given India for long time did not have the capacity to absorb their skills and give them the same salary for it that the West could....so at least this way India gets some earning of its best talent and also developed a large capable, wealthy (30 million strong) diaspora (this also has helped geopolitically).
Increasingly now there is reverse brain drain too, where Indian diaspora increasingly return or help to fund startups or join Indian corporates in new, growing sectors. Philippines has studied Indian trend in this paper sometimes known as "Reverse Brain Drain":
www.fsi.gov.ph
This is value addition in a way since these folks got career and industrial experience in West that they would not have got at the time in India if they stayed back then, so they are in effect returning "with interest" you can say.
The large part that will help to establish this trend even more are things like ease of doing business like I gave before:
en.wikipedia.org
As you can see as recently as 2015, India was ranked as bad as 142nd (worse than Uganda then).
As of 2020, in just 5 years time, it is now ranked 63rd...just few ranks below Italy.
This has been major accomplishment of this govt (they are pushing to get in top 50 ASAP) and we will see good results of this push in this decade materialising I think.