The number of bulbs that can be connected to the transformer depends on the transformer’s wattage. The higher wattage will allow the transformer to handle more bulbs. To know how many bulbs you can connect, calculate the total wattage of the bulbs. If you have 10 bulbs with 20 Watts each, then the total wattage is 20 x 10, which is 200Watts. Therefore if you want to power all of these, you should buy a transformer with a higher rating, say of about 300 watts.