Stick with one for now, especially if you are a new turtle owner. Baby sliders grow really fast and before you know it, they will be fighting in the tank. I have witnessed this many times in the past and it is extremely frustrating as an owner to have to deal with it.
RES's are the most aggressive form of "painted"-style turtle and they are VERY territorial. If there is another turtle around, they can fight to the point where they hurt each other. I used to have 2 RES's in the tank and the fighting would literally wake the dead. I had to dry dock one of them for 8 hours to have peace because I couldn't keep them in the same tank during the day. At night, because it was dark and they were sleeping it was fine. But I would have to get up early to separate them EVERY day. It sucked if I didn't make it there fast enough and when one turtle came up with a dislocated arm, broken tail, bite marks, etc. Don't even get me started on when the female started laying eggs too and the male would do the mating dance on her till she bit his long nails off.
Trust me, start with one.
