You’d need a counter, but s2Member doesn’t have one. You could create save an option in WordPress with your count, and add to it with ever new free registration using the registration notification, I guess.
So your notification would load a custom script, that loads WP and then you update the counter. Then, where you have the free registration form to signup at the higher level, before showing it you check the counter and if it’s higher than 200, then stop giving the upgrade and just register them at level 0 as usual.
These would help:
[hilite path]Dashboard -› s2Member® -› API / Notifications -› Registration Notifications[/hilite]
Knowledge Base » Hacking s2Member® Via Hooks/Filters
http://adambrown.info/p/wp_hooks/hook/wp_loaded
[hilite path]Dashboard -› s2Member® -› PayPal® Pro Forms -› Free Registration Forms[/hilite]
http://codex.wordpress.org/Function_Reference/add_option
http://codex.wordpress.org/Function_Reference/get_option
http://codex.wordpress.org/Function_Reference/update_option
[hilite path]Dashboard -› s2Member® -› API / Scripting -› Advanced PHP Conditionals[/hilite]