Along the lines of what ‘ j whitis ‘ said. if you are using incandescent lamps, find out the voltage of your lights and also the current rating of your lamps, (by the way, they should all be the same type of lights and the same voltage when they are wired in parallel). With all lights being the same voltage, what you then need to do is to figure the total current required. To figure the total current, you just add all the current rating of each lamp. With that information your then look for a transformer that will match with your A.C.mains on it’s primary side and has the same voltage out on the secondary side. You need to make sure that the secondary side is rated at a higher current amount than the total current of all your lamps together. By having a higher current rating than your total current, will allow you to add a few more lamps as needed.

Note: if you are using L.E.D.s for your lights, the procedure is a bit different because you will need Direct Current to power the L.E.D.s. What you need to do then is to use a D.C. power supply. Again, all your L.E.D.s should be of the same voltage if possible, and current rating. Then add the total current of all L.E.D.s. When you have that figured, look for a power supply that has a D.C. output that matches your L.E.D.s and has a higher current output than what you need, again so that you can add more lights later as needed. You may have to add a voltage dropping resister on each L.E.D. to match the output of the power supply. If using L.E.D.s, it would be wise to talk to your friendly and local electronics technician for help.