Hi,
If you’ve not discovered this yet and given the standard Recently Active Members Block Widget, if you have the Settings button enabled/active, you will be able to change the settings, e.g.:

If you are looking for the Legacy Widget version then you will have to install and activate the BP Classic plugin.
Additionally, 15 is the default number of members. As for the changing number of members after some period of time, I’m not able, at the moment, to duplicate what your are experiencing, meaning, given the default of 15 members I’ve yet to see the displayed number decrease from 15 and I’ve been observing for over a week. I’m just saying, I’m yet to experience the anomaly that you have described.
Furthermore, the data is retrieved from the bp_activity database table for the rows that have the component column equal to “members“, the type column equal to “last_activity“, as well as other criteria, e.g. number of members to return based on the number that is stipulated.