IMO, I don't base it on how many nodes you have. I base it on the number of hives, assuming the majority of the aliens are playing as they 'should'. In other words, skulks, lerks, and fades are keeping the marines busy.
First off - only one gorge, no matter how many people are playing. That lone gorge goes around capping nodes on the way to the second hive.
Once the second hive is up, this part of my 'theory' depends on the number of alien players:
6+, another player can go gorge
6-, stay with one gorge (
and maybe allow 'temporary gorges')
Once the third hive is up, I figure anyone can play as whatever they want. If you've got three hives, hopefully the marines aren't getting out of hand. If the whole team wants to do a gorge health-spray rush, let 'em.

This way usually works pretty well. In larger games, it's usually easy to request "only one gorge per hive". It gives that one gorge lots of resources to support the rest of the team, and also provides a larger number of attackers.