There are two ways you can take attendance, both require PCO Check-Ins.
If you just want to take general attendance, where you tally the total number of people in a specific place at a specific time, you can use Headcounts, which by itself is free. You will need to sign up for PCO Check-Ins, but since it charges you based on the number of individual people checked in, and Headcounts doesn't actually check people in, it doesn't affect your pricing at all. In fact, if you only plan to use PCO Check-Ins for Headcounts, you can sign up for the free plan.
If you would like to take specific attendance for each person so you can know the names of the people who attended, and their attendance history, you can check them in.
To check people in you can set up a check-in station with someone running it, or set it up like a kiosk and allow people to check themselves in. In addition, you can also skip the station, take attendance some other way, and then an administrator can manually check people in afterwards. Either way, these all count as checkins and factor into your price.
Check-Ins packages are based on the the number of check-ins on your busiest day of the week. For most churches, you'll have the highest number of checkins on Sunday. Figure out how many total people will be checking in each Sunday and sign up for a package that fits. (You do not need to include Headcounts into this number.)