I am new to the HCA! I will soon be getting some hermit crabs!

I have a 20-gallon long tank without a lid, and I will be using that to house a few (maybe 2-3) Purple Pincher hermit crabs.
I will most likely be purchasing a mesh lid from Petsmart, and tampering with it so that it will hold in humidity. I will be using Coconut Fiber (Eco Earth) as the substrate. I'm actually going to try to own them on a budget which will be hard. I have a few questions:
1. How many hermit crabs can I keep in a 20-gallon long tank? (and what sizes)
2. Can I use only Eco Earth as the main substrate without sand?
3. Any tips on owning these? I've researched a bunch and if you have tips please let me know!
I think that's it...
Thanks for reading my post!