Lay lines run around the world, scars from the big invasion that kicked off reality. They work like water pipes. One guy turns on a facet screw you can't spell, he gets a lot of pressure, but not all of it. Likewise, one guy wouldn't be able to access all of a lay line just cut he's alone. Get 100 people on a pipe running faucets, won't have too much of an effect, everyone gets good water pressure. BUT, get 1000 or more people on it, the power will reach a point where it can't give everyone maximum pressure. So it divides it up, giving everyone power, just not very much.
Math example:
there are 100 units of power
each mage uses at max 10 units of power
1 mage alone uses 10 power, leaves 90
9 mages use 90 power, and all are at full power
11 mages, the power would have to be divided between them. Each will have slightly more than 9 units of power.