What is the acceptable amount of Orphaned Instances before you start to notice performance degradation of the BizTalk server?
Well, let me answer it this way. The acceptable number of orphaned instances is 0. Meaning, this shouldn't be happening, and if it is, it's a problem to solve.
The answer to your actual question is...it depends. Depends on your database and app profiles.
Make sure the SQL Jobs are all running without error, then identify the app that is creating the orphans and refactor it to not do that.