You want to use the fact that the temperature doesn't change. So for an ideal gas, p1v1=p2v2.
Use this to find the volume of helium that you can fill up the balloons with, and then use the volume of helium in one balloon to work out how many you can have.
An important thing to keep in mind is when emptying the cannister of helium, the pressure won't reduce to 0 rather 1atm( roughly 1.1*10^5 pa).