How many red ear slider turtles can live in a 10 gallon tank and is it possible to keep one in like a plastic container or tub if so how big should it be?

Only one, and sometimes a ten gallon tank isn't enough room. The tank should be 4-5 times the turtle length wise and 3-4 times the turtle in width. If you have more then one, each turtle should get that amount of space. The water should be 1.5 times deep for instance a 7 inch turtle would have water 10 inches deep.