Minimum 2 log groups. But advisable to have more since you might have a condition where one log group is still 'Active' when the other one is full and a switch is required. If more than one log groups consistently show an 'active' status in V$logfile, you need to add more.
No. of members in each group will depend upon no. of disk drives you have, since each member should be (not necessary) on seperate disks. I think 3 members per log group should suffice for multiplexing.