For basic lap counting, your finger! These both work off the numbers on the keyboard.
For better, automatic lap counting, they will need some type of transponder system, such as the AMBrc transponder system. They tend to be expensive (although there are some smaller ones out there. . .)
But I've seen a lot of races counted with only finger counting - SoCal Raceway counts all of its off-road racing by finger.
RC10L2.5W - RC12.4 - RCNTC3(bmi) - TC4 (modded) - B44.2 - plus rent-a-rides! :D