Will you be dividing it roughly in half ? I'm not certain of the adult size of a platy, but you take the amount of room they'll have and divide that by the adult size of a platy, and that's how many you can have.
So .... if you're giving them 7.5 gallons of space (half of 15 gallon tank) and the adult size of a platy is 2 inches (I'm guessing, not sure if it's even close) then you can have:
7.5 gallons / 2 inches = 4 <--- well since you can't have 3.78923 fish in a tank.
Check on the adult size of a platy and go from there, hope this helps.
Also, make sure whatever you divide the tank with has sufficient holes to allow the water to circulate well, unless you're planning on running a filter on both sides. Remember also that bettas don't like a lot of current, so if you're running 1 filter only, put it on the platies side. And one last thing ..... if the divider is clear, make sure the betta doesn't wear himself out flaring at the platies.
I've also heard that a betta *might* be ok in a community tank with platies. That's a big MIGHT. You can watch his reaction through the divider and see how it goes, and then remove the divider if you decide it looks safe enough to leave them together. Just always be ready to put the divider back in at a moment's notice in case something does happen.