I saw it mentioned somewhere on here that FB doesn't like it or allow it but it seems ridiculous that you could be an agency and only be allowed to have 1 client at a time.
Has anyone ever lost an ad account for having advertiser access and running ads to more than 1 client account?
I would just get an agency/grey account for every client through their small business program, it doesn't cost anything, it spreads the risk however small it is and you get a bunch of fb ad accounts which might be useful some day
You can add admins to accounts just fine... And then all accounts appear in your ad center. Most agencies will use PMDs though so it doesn't really matter.