AMEX cards simply break down into credit cards and charge cards, while most of the cards are credit cards, but the Platinum/gold/green are charge cards, AMEX allows you to have 4 ( 5 if good credit standing) and 10 charge cards under 1 SSN.
In theory, AMEX opening bonus only once a lifetime on any card, but there are some exceptions, especially on small business charge cards, as well as the Delta family of cards, may offers promotions that are exempted from that rule, from time to time, and some cards do upgrade offers.
I just closed 1 platinum business card and now Amex offering for me to open another Biz Platinum for 155k points, so keep your eyes on your log-on screen, if you get an offer like this, you can go another round.
On your log-on screen, sometimes you get pop-ups, or under offers, or on the right screen panel, like this.
