There are no design limits, but there are some some practical limits. Greater than 32767 individual shapes gets into a grey area (but is still useful); increasing to more than 65535 shapes may cause more problems.

There are various string-length limits on the command line an in ASCII region files. There's no hard and fast rule, but this general statement holds: if you think the problem is due to an exceptionally long line or string length, it probably is. This is usually seen with many, many sided polygons, which are best input as FITS regions since they don't have this kind of limit.