If you would like to know how many number of sheets are in a stack, you can calculate this using the following steps:
2 1/4 = 9/4
2 1/4 inches / 3/16 inch = 9/4 / 3/16 = 9/4 * 16/3 = 3*4 = 12 sheets
The correct result would be: 12 sheets are in a stack.