It doesn't matter and to be honest, it's difficult to tell the difference...I'm about 3" ovestocked...If you don;t mind really keeping on top of your tank maintenance, you may want to think about getting two......It would be for their benefit to keep more than one. Otos are not very active. While they do produce waste, they use less oxygen then more active fish.
Also, when choosing, be sure to select ones with little round bellies....This would indicate that they've been eating. The most critical point of survival with Otos is generally the first 2-3 weeks... Be sure your water parameters are correct and make sure you
acclimate them to your water properly before adding them to your tank. Don't be surprised if they hide or set stationary in one spot for long periods of time at first. If you don't have algae in your tank it might be difficult to get them to eat. Some don't take readily to wafers or other fresh vegetables. I waited until I had a good algae base before getting mine. Now they're happy as can be with constantly full bellies!