They're limited by the stash storage space and more practically by how long you want to wait for the loader routine to show the list. Each stash is 78788 bytes, or about 100KB. The default stash space is 64MB, minus what is used for configuration backup. One meg stores about 10 stashes, so the soft limit is ~64,000 stashes. If you run out you can always move the stash path to a USB stick or expand the stash area, but I've never tried either of those.