Use
(Get-AdGroup -Filter 'Name -eq "<NAME OF GROUP>"' | Get-AdGroupMember).count
This will give you the amount of people in the group. I believe there is a limitation to the Get-ADGroupMember which is around 5000, if you think you will reach this limit please check here:
https://stackoverflow.com/questions/46078880/get-adgroupmember-the-size-limit-for-this-request-was-exceeded