Minecraft wiki entry for Units_of_measure, associated text: 1 block = 9 items. Used for resources that can be crafted into storage blocks and back (most kinds of processed minerals, slimeballs, bone meal, melon slices, wheat, dried kelp)
1 stack = 64 items = 7 blocks, 1 item. Note that some items like banners, signs, eggs, and ender pearls only stack to 16. So if someone says they have a stack of eggs, they have 16. For unstackable items, a stack is just 1. So saying things like "a stack of pickaxes" to mean "64 pickaxes" is best avoided. Simply say "64 pickaxes".