How Deep Should You Actually Go
2–3 inches is the right depth for most beds. Less than 2 inches doesn't suppress weeds or hold moisture well, but piling on more than 3–4 inches can suffocate roots and hold too much moisture against the trunk or stems, which invites rot and pests instead of preventing them.
Keep it off the trunk. Mulch piled directly against a tree trunk or plant stem (the "mulch volcano" look) traps moisture against the bark, which encourages rot and gives insects a place to hide. Leave a few inches of bare soil right around the base.
Bulk is almost always cheaper past a few cubic yards, same as with soil — most landscape suppliers price mulch by the yard specifically because bags become expensive fast once you're covering more than one or two mid-sized beds.