1" = 10 gallon isn't the only guideline used. Another popular one is that your turt's shell is Z, so tank length is 5xZ, width is 2xZ and water level is 1.5xZ - this may be considered an absolute minimum.
As for multiple turts, 1" = 10 gallons becomes a luxury. The larger tank will help keep aggression and harassment down. Don't forget the increase of waste that needs to be addressed. Anyway, I think 1" = 10 is good for 1 or 2 RES. I personally like the guideline because I think it makes people think about housing multiple turtles.





