My understanding is that you do NOT need to carry a spare set of bulbs.
In the changes to Spanish traffic law that took place last year they removed this requirement altogether.
When it was included in the list of requirements it was a good idea - but that was when bulbs could be easily changed at the side of the road, and there were only a dozen or so different types of bulbs used anyway.
These days there must be hundreds of different types of bulb on the market, and maybe even four or five different ones that you would have to be able to carry just incase one blew on your car. Even then, changing a bulb on a newer car would be almost impossible at the side of the road because it is now so difficult - assuming that it is not a 'sealed unit' type headlamp.
A lot of people made a mockery of the law by simply carrying one bulb in their glove box, that didn't necessarily always fit any of the lights on the car anyway.
Fortunately common sense seems to have prevailed now and the law-makers have realised that it is an out-dated requirement.