Level 6 has no limits.
But from what i understand, you concern is how much clients it supports while running bellow 100%.
Well, this is a very tricky answer, it depends on the config of the RB1200 and the BW that you give to each customer.
Router performance normally is measured in packets per second, and if you go to the RouterBOARD page you can see that for router it does about 117.000 pps for big downloads (MTU=1518), and this is quite allot for it´s price.
For a simple iperf tcp test between to linux machines with RB1200 in the middle, with routing and simple firewall rules (connection state evaluation), it does about 680Mbits of throughput.
But for a small difference in price, you have RB1100AH that almost doubles the performance of RB1200 (208.500 pps)