Hi,
Like most singing bass players who have been doing this as long as I have, I can probably sing 40-50 songs off the top of my head. I've got about 300 in my lyric book that I could sing if necessary, or by request. Plus I know many more than that well enough to play. But my brain is just about full and can hardly hold any more notes and words. I have to forget something old in order to learn something new.
Add what I know to band members of the same vintage and we probably could pull off 1000 songs between us. But for the younger folks, they've really had to cram a lot of music into their brains in a short amount of time, or they have deep books plus excellent skills, to be able to pull of such a vast repertoire. It is fairly common among working (cover) musicians who've been doing it as their profession for a while. But I would agree a good number of musicians have a great talent, fantastic memories, are expert at just knowing a very large number of cover tunes, and are able to pull them out at a moments notice. I wish I was one of them.
I do recall first starting out trying to get to that magic number (of songs) so that we could play a three hour after-school dance and not have to repeat a song. I think that number was 40, twelve songs per set plus a few extra just in case. Now I sometimes struggle trying to remember the words to songs I've been singing for 20 years. Sometimes it's not fun getting old. The first thing to go is..., um, the first thing to go is...., oh, I forgot.
Practice makes perfect and repetition is the key.
Thank you for your indulgence,
BassCliff