we are developing an app which will allow an admin to get managerial permissions to access users' advertising accounts on Facebook. But it looks like there is a limit to 25 accounts only.
Ads Management Basic Access lets your app access the Facebook Ads Management API on up to 25 ad accounts. The basic access level is meant for businesses to scale out their use of the Ads Management API to a limited set of initial customers. source
How can we get lift this limit? We found here that
„Q. Will I run into problems adding more than 25 ad accounts to a user? A: No. Adding ad accounts to a user via the /userpermissions API will bypass this limit. More information available here.”
But what does it really mean? Will there be any limits on, for example getting stats of campaigns for those users above 25 limit?
I think you're confusing two similarly described but unrelated things -
There's a limit when your app has Basic access to the Marketing API of how many accounts your app can manage through the API, and there was another overall limit for how many ad accounts a user can be the admin of (which was 25, but when using Business manager that no longer applies)
For your app to manage more than 25 accounts via the API, you need to submit your app to Facebook for approval to use the Marketing APIs at the Standard level, there's a link to the form for doing so here: https://developers.facebook.com/docs/marketing-api/access#standard