20 - I have this exact mess, eleven profiles, and I still do not think it is an app.
The switching is not the annoying part. The annoying part is that I cannot tell which of the eleven still has a balance without opening eleven different providers' apps, several of which no longer exist. That is a data problem, and none of the providers will give you an API for it because the unused balance is the business model.
You would end up scraping confirmation emails, which works right up until it does not, and then your app is silently wrong about money. I would not build it. I would like someone else to build it and I would pay for it, which is usually a sign that the thing is harder than it looks from the outside.