So far as I understand it, everyone gets one support spot. Then if they are gentry they get another, if they are in a guild they get another, if they GL they get another, etc.
If you go the guildless route, you're basically left to one support slot.
Is it possible to expand this, so if you're on roster, you're able to support two people? Essentially, you'd be running an IG business which is comparable to anything the Merchants are doing, and would even be plugged into that same system of traders.