There's a comment in s3.route53.readme that says: "Also limits of 100 domains and 100 records, though both limits can easily be removed." I couldn't figure out why the last part was there. So I wrote a test script to create 150 new records in davecast.org.
The records appear to have been created correctly. Now let's make sure they resolve correctly. (They do.)
So I'd say that at least for creating, there is no limit of 100 records