It all depends on how many you need. If you are asking for SEO purposes, then it depends on how much SEO resources you have to dedicate for different websites. Having a website sitting online does not help you much..
Most of our clients have 1 website. Our clients who are more serious about their web presents and receive a good amount of traffic from online have 2 or 3 websites. From what we can see, having 2 or 3 websites is starting to become common.
This Google Custom search may do a better job of searching the forums for some keywords than the old forum search does. The results do not include threads from the Asset Managers Forum however. To search that forum you will need to be actually in the Asset Managers Forum and you will need to use the old forum search below.