that should be enough for 500+ rigs easy... mining doesn't take up much. More lines is a good idea to be sure you're never without internet.
I know people that run 3000+ GPU miners on a 100/100 Mbit line.
Yeah no wonder, i even wanted to put 1000+ rigs at start ahah, really bandwith isn't an issue, but latency (optical fiber very good for that) and redundancy for mining uptime are good, better take 3x100MB lines that 1x300MB line
