There isn't a hard limit per se, however, all review items go into a specific review item table to keep track of decisions, and the reviewers and the items.  When you have lots of concurrent reviews open, that table can get overwhelmed depending on the performance of your DB server.   

There is the classic answers of adding RAM, increasing disk performance, attempting to index it differently.   I have a rule of thumb that reviews with more than 50k items are just outlawed, and in general I've seen success with an Oracle backend and 200-300 reviews, with an average size of a couple hundred review items each.  Its when I've had much larger reviews (10,000+ items) where things start to hurt.

It may be worth a conversation with your DBA team to see if they have any performance metric tools, and if they have any ideas on improving performance.  That may lead to a conversation with the business about hardware investment.   The efficacy of your DBA team will be tested.

--Jim