VMWARE vCenter DB is 9GB in size after a few months 15 hosts 500 VMs , is this normal?

It depends on how long you keep the events for.  Take a look at retention under Administration Settings.  By default the check boxes are unchecked and we set it to 60 days.  You should also check what level of logging you're doing, Level 3 and 4 will give you more information but will take more space

Just make sure that you're running full blown SQL instance, SQL Express has a 10GB limit.   You should also setup maintenance plan for the DB and Logs.

What size is the event table?

SQL Express is too small for your environment upgrade to SQL full.

Or switch to VCenter Appliance which is better.

VMware support have stipulated that SQL Express is no longer suitable for large installations as far more tables and information is now written to the database.

1. Check table sizes with all your current issues and changes I would expect the events table to be massive this can be dropped and DB reduced.

2. Change and keep events for 7 to 14 days

3. Or upgrade to SQL Full OR

4. VCenter Appliance and Postgress Database now can handle larger environment s