If you have a Fungal Behemoth with 2 +1/+1 counters on it and no other creature with any +1/+1 counters on it, is the fun guy a 4/4? The two counters give it a base power and toughness of 2/2 and then those very same counters give it another +2/+2. I'm pretty sure that is how it works, just needed confirmation.
Yes, that's correct, +1/+1 counters placed on Fungal Behemoth are essentially doubled up in effectiveness.
- Each +1/+1 counter on Fungal Behemoth gives it +1/+1, as normal.
- Fungal Behemoth additionally has an ability that counts the number of +1/+1 counters on your creatures, and gives it +1/+1 for each of those.
These both happen cumulatively, and the ability considers even counters placed on the fun guy itself. So each +1/+1 counter placed on the Fungal Behemoth gives it +2/+2 — once from the counter itself, once from its ability.
This is also described in a ruling on Fungal Behemoth:
Any +1/+1 counters on Fungal Behemoth itself count toward its base power and toughness, then give it a power and toughness bonus. For example, if there are two +1/+1 counters on Fungal Behemoth and three on other creatures you control, Fungal Behemoth is a 7/7 creature.