Here are some interesting data about how many Spaniards who live in the relevant regions speak or just understand Galician, Basque, or Catalan. The Spanish government asks about language usage every time it does a national census. Unfortunately, it still hasn’t processed the language results from the most recent census (2011), so these numbers date from 2001.
At that time, the regional languages were spoken by over half the residents of their respective territories: Basque by just over one-half of residents, Catalan by about two-thirds, and Galician by almost all. The lower numbers for Basque are to be expected because, unlike Galician and Catalan, Basque is unrelated to Castilian Spanish and therefore difficult for other Spaniards to learn. (This difference also explains the large numbers of Spaniards who can understand, though not speak, Galician and Catalan.) The higher numbers for Galician, compared to Catalan, are probably attributable to demography. As shown in the last column, over 90% of Galicians were born in Galicia, compared to less than 70% in the prosperous and cosmopolitan Catalan region.
It will be interesting to see how the 2011 data compare.