In my opinion, I think 300 collectible coins in total are enough. Let's me explain why I choose that figure.

(1) 50 or 100 coins are very limited, and the forum has so many prominent users as well as staffs. The limitation on 50 or 100 will prevent good users to get them. If we look at BPIP, most merited profile page, most of users in top 50 or even in top 100 are very prominent users. There are very little new users get spots in top-50 or top-100.
(2) 300 will give opportunities to good users, who have been here long enough to have significant contributions, but neighther have had long history enough nor significant contributions to be competitive compared to extremely prominent users and staffs.
(3) Such collectible coins (if new users can get) will play as good proof of motivation, and newbies will be inspired from it.
Newbies are sources of future growth of the forum. Let's give them motivation.