The fighters are clearly from the Republic era so it's possible they were salvaged in bulk.

Biggs proves they are trying to recruit pilots so it makes sense that they could end up with more ships than pilots.

There are certainly a lot of X-wings and Y-wings in ROTJ despite all the loses in the other films.

If the Yavin base holds all the X-Wings and only two came back where are all the other ancient X-Wings coming from?

Either they had lots of bases full of these things (in which case why is Tarkin so keen just to find a Singular base?) or they had one with more ships than pilots.