The asynchronous mod-12 counter is shown below:
Mod-12 counters are designed to count states triggered by an input clock signal. The Mod-12 counter counts 12 clock signals from 1 to 12, then returns back to 1 and starts counting to 12 again. Mod-N counters with number of states @$N=2^n@$ require @$n@$triggers, mod counters with the number of states that can't be expressed by this formula, require using logical elements also.
The word asynchronous means the data bits states change one after another independently on the input clock signal (shown on the left). Synchronous counters change states at a time synchronously with the input clock signal.