Microsoft seems not to mention a specific limit for keys or certs, but for databases it is 32767 user databases per instance.
If you consider the TDE key hierarchy all databases have their own DEK, so it's naturally to assume that at least one (and with rollover) at least two keys are possible. (It would not make sense to specify a maximum for databases if they can't use all features like a DEK).
The DEKs might all be encrypted with a single database master certificate (or multiple, but you would not have one for each DB. But even if you would, since those are just stored in master database tables, I would not see a problem (besides heavy CPU usage on startup).