DB2 DBM paramter NUMDB has value 8. Should I change it to 1 because I have only one database?
Information Center is pretty reliable, and I don't see any indication that it will divide memory for non-existent databases. Do you have another link that indicates that?

Otherwise, it is meant to be the maximum number of allowable activated/connected to databases. So there is no need to set it high (I believe by default it is set to something like 12).

Based on the page you have, it is recommended that you set it to the number of databases you have + 10% for room for growth. So if you have 1 database, you would want 1 database + 10% (which in this case would round up) so you would set it to 2.

The reason I believe IBM recommends this, is that you can sometimes get caught with NUMDB less than your number of databases. We have had this happen to us on several occasions in our development and QAT environments. We set the number of databases to something like 9 or 10. And then the developers decided they needed another database. We DBA's ran the CREATE DATABASE script like usual and it didn't fail. However, when the developers tried to access the new database, they got errors (I don't remember the exact SQLCODE off the top of my head). And it turned out we needed to up the value of NUMDB as we had reached the maximum number of activated/connected databases.

So I would follow IBM's recommendation, which in your case would be to set NUMDB to 2. This gives you what you need with some room for growth.

I think this also might not be a "hot" setting, thus requiring you to run db2stop and db2start for it to take affect.