Depending on the setup you have. If you are willing to do alot of water changes and keep a close eye on the fish I would say it's safe to say as many as 4 But really a 75 Gallon or larger would be better for the Red Bellies. A Rhom wiill be fine in a 75 Gallon tank for years, however if you can keep him alive long enough there will come a time when it will make sense to you to put him in a 125G or larger tank. I would suggest something maybe a bit wider that a 125 Gallon. The usual 18in isn't all that wide. Good luck with whatever you choose.
if you got a small rhom (3" or so) then u could keep it in the 55g tank for quite some time since they grow rather slow....probably until the rhom reached a size of 8-9 inches in total length. I wouldnt keep him in there past him reaching that length just because he would be kind of cramped.
also, if u went with the '3 reds in 55g' then u would be able to keep them in that tank for life...you would have to upgrade to a larger tank at some point.
Id have to agree with mack! 3 ps are better than 2 but for life? Nothing lasts for life, why should a tank. Always be ready to upgrade or stay out of the hobby. If u had a corvette as your daily driver, and u got married and had a kid or more wouldnt u upgrade cars? (I wouldnt trade a corvette though) But u would need a 4 seater+ right? So think of it that way! good luck
If you literally mean the life of the fish, none can live in the 55. 3-4 can go in the 75. If you are talking the literal life of a rhom, then a 75 wont be big enough. It will, however, last a very long time before it would be necessary to upgrade.
Now, advice you didnt ask for. You could put three reds in a 55 gal and they will be fine in there for quite a while. You could buy a rhom at almost any size, under 12" lets say, and you could keep it in the 75 for quite a long while. This advice is probably more realistic for you to follow unless you plan on keeping your piranhas for years and years, and want to be set up for life from the get go.