There's no hard and fast rule for a cleanup crew, especially in a smaller tank. While it may be tempting to add a bunch to clean up the
diatoms and algae, you'll be better off starting slow and adding more snails and crabs gradually to find a good level.
I'd go with maybe 2 astreas, 2 turbans, maybe a nasarious and a turbo to start., and maybe 5 or 6 dwarf hermits.
If you add the hermits, it may be necessary to add a small amount of fish food (pellet, preferably) once or wice a week for them to eat until there are fish in the tank. They can then scavenge on their own.
As your tank matures and changes, you can add or remove different CUC members t o suit your current conditions.
My suggestions are by no means gospel. In fact, I have always had pretty small CUCs in my tanks, so I'd encourage you to get a bunch of opinions and sort of make up your own mind about what species and how many of what you should add.
