This is another of those simple questions that we get a lot. Getting a number is easy, you just walk into a provider’s store, show your ID and get a SIM within minutes. The tough question is which company to choose?
This will depend a lot on your requirements such as do you need internet in your home too? Do you need cable TV? Do you want to get a contract number or a prepaid one?
The largest companies in Spain are Movistar, Orange and Vodafone all of which offer either mobile numbers only or packs which include Fiber, Landline, Mobile and TV.
We will not get into pricings here because those change sometimes daily but overall if you want everything you will have better options with either Movistar or Orange and they even allow you to configure your packets depending on your requirements to a greater degree.
If you just want a mobile number or mobile + fiber you have a lot more options such as lowi.es, tuenti.es, pepephone.es, simyo.es etc.. Unfortunately the majority only offer the option to get the number online as a way to cut costs and will mail it to you so they are not the best option if you want it as soon as possible though.
To sum up, if you just want a number or number + fiber, go with the smaller companies as they offer a more competitive price. If you want full packages or need the SIM card as soon as possible the best options are the larger ones.