The best theory I have heard is that worm hole systems are organized into constellations. Each constellation has a limited number of each type of exploration site. When these sites are cleared they spawn randomly within the constellation.
If they randomly spawn in an unused system they will stay and extended period...perhaps until someone actually enters the site ,starting a 72 hour timer, then clears it or the 72hrs pass.
The underused systems will eventually collect a large share of the sites.
So the number of sites have more to do with how actively the exploration sites are cleared from that worm hole system and the activity level of the worm hole systems in the constellation.
I have lived in Wormhole systems for 6 months and more with 2-11 POSes at a given time. I can't offer any scientific proof but I saw no difference in spawn rates
For some constellation info:http://www.staticmapper.com/index.php
http://evemaps.dotlan.net/system/J100806 (for an example of a specific WH system)