After setting up the connection between your Figure account and your Campaign Monitor account (which you can read about here - Marketing) the first sync should show identical numbers of clients in each place. So if you have 500 customers in Figure there should be 500 in Campaign Monitor. 

After a few campaigns and syncs, the numbers will start to differentiate. This is because as customers unsubscribe or their emails bounce, Campaign Monitor will suppress those email addresses, and essentially count them as inactive. Below is an example of what your list in Campaign Monitor will start to show after months and years of activity.

So you may have over 5000 customers in your Figure account, but Campaign Monitor will only be showing the active email addresses, while the Unsubscribes and Bounced emails get set aside.

Let us know if this doesn't make sense.