I have a 15 amp circuit dedicated to low voltage lighting. Each transformer delivers 300 watts. How many can I connect and use at the same time?

Volts X Amps equals Watts 120 X 15 + 1800 Watts
Derate by 80%(1800 X .8 = 1440 watts) Maximum you can use, will probably be on for more than 3 hours.
4 is maximum I would use.