at the most 1 pair, but a 90 may eventually be too small for a pair. males can get rather large like 14-16" and I've heard even larger. the largest one I've ever seen though was around 14". you should be able to keep a mated pair for several years though before you may see the need to upgrade them to a larger tank. if you want a pair it's best to get several (6 or so) juveniles, grow them up together and let them pair off naturally. once a mated pair forms you'll need to remove the extras or risk them being killed.