1. All you can find in UCS database (see the UCS database model document
2. All is stored in UCS database at this moment. There is no application limit on that, just the database limits can be reached. Of course, the other limit regarding to this can be on index level (Apache Lucene) which is embedded in UCS.
3. Is pretty same as above - just possible index limitation or database limits.
For better understanding how UCS works in general follow the available documentation (for example
https://docs.genesys.com/Documentation/ES/8.5.1/Admin/UCS)HTH!