These bulbs are meant to be ran in series so that if one bulb is unscrewed or burns out, all of the other bulbs turn off. The following calculation is based on your arch using 120V (not 220V) and the bulbs are wired in series. How do you know if it is 120V - it will plug directly into your wall outlet, there won't be any transformer. How do you know if it is wired in series - if you remove one bulb they all go out.
So to figure the correct bulb, you will need to know how many bulbs there are, once you know that it's easy:
Take the number 120 (which is 120V), divide it by the number of bulbs the arch has.
So if your arch has 10 bulbs - 120V/10 BULBS = 12V BULB, then you will need a 12V bulb.
What if the number doesn't divide out to a whole number like in this example: you had 8 bulbs, so that would be 120V/8 Bulbs = 15V, in this case go with the nearest larger number bulb, so that would be 16V.