I used to have 8 plecos in my 60 gallon tank. The trick is not to have more than one of any species and keep plecos of varying maturity rates. Some plecos, such as mature commons, can be very territorial. Royals become very territorial as they mature but when they are young you can keep multiples any species. To keep multiple plecos just remember one thing; build lots of caves. I would create at least 12 hiding places for my 8 plecos and of course they would fight over the best spot. The hostilities don't last very long. If you have an overly aggressive pleco that constantly wants to fight, get rid of it right away. I also kept relatively small plecos, most never grew over 6 inches.
My biggest problem with having that many plecos was that I had $400 worth of fish I almost never got to see.
All I can say is plecos are awesome, get as many as you can. Also remember they aren’t all algae eaters, provide them with wood and shrimp pellets depending on the species.