There is no limit to the number of members registrations and concurrent users the BuddyBoss platform can handle. The limit will depend on what your server hosting can handle.

We recommend investing in a solid server setup. With users interacting, you will need a reliable server to handle all DB queries and keep your site optimized. We also advise growing your server resources as traffic increases. Having a good CPU and RAM also helps as there are many database requests if the site is active.

You can check this link and see the minimum server requirements recommended for sites running BuddyBoss smoothly and with all functionality working as intended: 
https://www.buddyboss.com/resources/docs/advanced-setup/server-requirements/