Amex has gotten stingier about churning their own cards. For example, I got a Platinum card this year and was denied a signup bonus because I had a Plat card in the past. (For reasons not worth getting into, I chose not to argue.)
However, I'm pretty sure there is no per se limit on cobranded cards. If you get a SPG and a Hilton Amex, it should not affect your bonus possibilities on a Citi AA Amex, or a regular Amex, etc. I've never seen anything like that.