Ok, according to my understanding, one .sav, as long as the sram on the cart is 128K, can fit around 16-18 average songs.
Derp is best because littleFM can pull some sram trickery and compress multiple .savs onto a single cart. Your memory would basically be infinite, but I'd still recommend frequent backups, so if you don't need more than 20 songs on the cart at any one time, you could save some cash and get a bennvenn or a USB64M.
BennVenn(64) used to only support 4 32K .savs for LSDJ, which meant one song per save, which meant 4 songs total. But it recently got updated and now uses the entire 128K instead of splitting it up into chunks, so that means you've got the normal 16-18 song limit on one of those for half the price of a derp.
USB64M has the same 128K, and the same 18-or-so song limit as the bennvenn. There's two "pages", but that's just how the multirom works, the ROM memory and the SRAM memory are completely separate and shouldn't be mixed up. So, with USB64M, you can put one rom on page 1, and the rest (saveless games/programs) on page 2.
You can put multiple roms on bennvenn, too, but I don't know if it supports multirom out of the box, or if you have to manually patch all of your roms into one big rom file, or how the sram fragmentation works, or what. It's a relatively new cartridge, and I don't own one (yet).
Feel free to point out any misconceptions I may have, or add to this body of information. I think we need more info about all of the available cartridges in a centralized post somewhere. There's also the GB_everdrive(how do it's savs work?), the kk_gb_8m, and some SD card carts floating around/in development; and I'd love to know more.