My question remains: Once I have several wallets, this number right here (the one that says 100,000 bytes) shows only the holdings of the selected wallet
how do I check the total amount of GB and blackbytes on all wallets?
GB you can check in
http://explorer.byteball.org/, for blackbytes you'll need to check each individual wallet. And, after receiving them, you'll need to create a new backup.
also is there a limit of wallets you can create? (hopefully not..?)
No.
But since you're into privacy for 60 different addresses: how would you ever use the bytes? Sending them to the same address once again links them together.