Monitor number of users that accesses a certain app in Appian?

Refer login-audit.csv file from logs

I don't think we can segregate the end user access to Appian by application from logs.

If your application stores user information in DB when user performs any activity , then you can query the application's DB tables to identify the users.
E.g createdby, updatedby fields or task history table storing information of all tasks that are completed or any related action performed etc.

If you have multiple applications deployed, the login-audit.csv will not give you the answer you need, but rather only who accessed the Appian instance. One way is to build a write node to log access to individual apps separately where you have your own built log.

Another more indirect workaround is to access the DB and utilize a record that was written/updated associated with that app. I don't recommend this as it's inaccurate and only would return results if the user performs a write to the DB.