What is the max number of retainers?

[Jojoya]

Some who is already spending $14 a month for the extra 7 retainers and plans to spend another $5 for the convenience of being able to transfer items between retainers probably isn't going to balk at spending another $2 to get the last retainer that will then be available to them.

Those who do have a problem with it aren't likely to be spending the money in the first place since it's not needed to play the game.

[Seraphor]

Some who is already spending $14 a month for the extra 7 retainers and plans to spend another $5 for the convenience of being able to transfer items between retainers probably isn't going to balk at spending another $2 to get the last retainer that will then be available to them.

Those who do have a problem with it aren't likely to be spending the money in the first place since it's not needed to play the game.

Funny thing is, I might have been more inclined to buy pay for premium on this app if it was $7 and included an extra retainer.
E.g. I have 3 retainers, this would boost me up to 4.
SE would probably sell more premiums that way too.

Like Mhaeric says, it's the principle of paying in order to have the opportunity to pay more that's the issue.