Quick answer
Most beds do best with 2 to 3 inches of mulch. Less won't block weeds or hold moisture well; more can smother roots. Keep mulch pulled back from stems and trunks.
How to Estimate
Multiply bed length by width in feet to get square feet. One cubic yard of mulch covers roughly 108 square feet at 3 inches deep, or about 162 square feet at 2 inches.
Avoid Volcano Mulching
Piling mulch against tree trunks traps moisture against bark and invites rot and pests. Leave a few inches of space around trunks.
Refreshing
Top up as mulch breaks down, usually once a year. Rake old mulch first so layers don't build up too deep.