BuddyPress has sites with more users so there is no physical limit, however the number of users a particular installation can support is due to a large number of factors including that actual usage profile of each of those users and how you have chosen t host your install. It’s true that the larger sites often have had to undertake some pretty significant customisations in order to support such a large user base.
Thanks for the reply.
BuddyPress + WordPress will work well when there are 500,000 users?
It is important to know before, otherwise I see already now an alternative route.
What important customizations you talking about? I’ve already been working on reducing CPU consumption etc.
It would be interesting to know why [your] developer told [you] that [your] site will not be able to manage more than 70,000 users.
Why precisely 70,000 ?
As @venutius already told you, there is no physical limit. And if you have the necessary cash to buy storage and bandwith, go ahead!
A common example for a large site running BuddyPress, is profiles.wordpress.org mentioned by lead dev John James Jacoby (in 2012). It is still the case in 2016.