You can keep track of how many modules your members have marked "Complete" by using the "Members Progress" feature.
There are two ways to do that:
Method #1: Go to the members page of your group and click on the Members Progress button:

For every completed module, you will see a green square. For example, in this case, all group members except for one completed 2 modules:

Please NOTE: the squares become green when a group member marked a module "Complete" using the checkbox at the bottom of each module.
Method #2: Go to the members page of your group and click on the "Completed Modules" icons next to each individual member to see which modules they completed:
You will then see which modules each member completed, for example:
