Try repairing the count from Tools->BuddyPress, if it does not fix the issue then it seems like your members have not yet logged-in on the site. As soon as they all will log in on the site the count will be fixed. Because BuddyPress shows the count of the active members(at least they logged in once).
Everytime I need to go back to admin section, tools -> buddypress.
Let me explain, as you are using Memberpress registration so you are not following BuddyPress user activation process which is ‘User registers on your site and then they get an activation mail where they get a link and by clicking that they activate their account and as soon as they log in the count gets updated.’
So here one can disable activation email and then can go to the Users screen and there can activate them manually and as soon as they log in there on the site they will be listed there.
Normally if people are not deactivating BuddyPress activation mail then I suggest them this plugin https://buddydev.com/plugins/bp-autologin-on-activation/ so that their users get logged-in automatically as soon as they activate their account and in this way, BuddyPress will receive it’s active members data and count will be fine there.
Right now it seems like you are having issues there due to Memberpress registration. I will suggest you auto-login them on registration and redirect them to their BuddyPress profile and in this way, you do not have to go and do that count fix again and again hopefully.