I think I read somewhere before about 1 in a 70g tank. That was after I had already put a second one in my 90g
. My first one went in when I was battling a bit of cyano and a mini cycle after a move and still hasn't grown as big as the second one. Both are coming up on two years now. They pop up every now and then (mainly early and late in the day when the lights are lower). I don't really do anything to my sand (also about 1.5") so I kinda like having them in there if they are keeping things somewhat mixed up in the sand bed.
Not sure if 24g might be a little too small. Mine used to climb the glass every now and then which may have meant they didn't have enough food in the sand bed. They haven't done that for about 6 months now.