The decision should depend on your available server bandwidth, etc. I usually run mine on 4, unless I want higher quality racing, in which case I plump for 5 (it is a "significant" increase).
It's entirely possible to run with 6 (given as fast enough line - for 47 guests and 32 racers that equates to 829KB/sec upload required - well in the reach of a lot of dedicated servers [most servers are at minimum 10Mb/sec which equates to 1280KB/sec]). This doesn't consider the cost of transfer, however, which is something you should consider. Just because you can transfer at 10Mb/sec permanently doesnt mean you should. This depends on how you are billed, etc.
However, I personally just wouldn't consider it worth while for an online server though, as you're subjected to UDP packets getting lost or arriving in a different order (meaning wasted packets), etc. This tends not to happen on a LAN, so it makes much more sense for packets to be thrown as fast as possible. For online racing 4 PPS is perfectly reasonable imho.
In short you can almost run 2 servers at 4 PPS for 1 running 6 PPS.