Currently there is a limit of 100 shares per pool (although you can have more in lowest pool, they won't all receive upvote value).

We are adjusting the member limits in the pools and will be transitioning to a limit of 125 in each pool. Since we currently have 8 pools... that's a theoretical limit of 800 to 1000 shares. Unless you're doing a lot of incentives really quickly, the limit is likely to increase before you reach it.