Just about all modern submersible heaters have a built in thermostat that will turn them off when they reach the set temperature, so no worries there. The only worry is that the built-in thermostat can be a little on the not-so-accurate side. If you want bullet proof, a heater that is plugged in to a separate temperature controller is a pretty rock solid way of keeping stable temps. But I'm not sure if stability is critical for you.
As far as how many, I'm a big fan of having 2 heaters in a tank. I size them so that the TOTAL Wattage of the two heaters is just a bit higher than the Wattage of a single heater that you'd put in there. So if you determine you need a 250W heater, I'd put in two 150W heaters. The thinking is that if one of the heater's thermostats sticks in the "on" position (which they normally do when they fail), with two smaller heaters it will take longer for the tank to get to dangerous temperatures and you'll have a better chance of noticing it before it does. With a single big heater, if it sticks on, your tank will be boiling before you know it. Same thinking goes for if the heater stops working - at least you have another one in there that will provide *some* heat.
Just for reference, in my 46g tank, I run 2x 100W heaters. Tank is kept at 79F, and the room gets around 64F at night. So I'm thinking 2x 250W is a little overkill for you, but it all depends on your room temperature.