You need to understand about 'bandwidth' and 'latency' first.
12 Mbps is the bandwidth that you have mentioned(most probably downlink).
This roughly means that 12 Mbs of data per second, can move in the network via your ISP towards you.
Now try to understand 'latency'. It's the time it takes for your data to reach from your end, to the server on the other side with which you are communicating.
Online gaming is not dependent so much on the 'bandwidth' as it depends on the 'latency'. Therefore your network connection needs to have a good latency.