How many bags of mortar per brick or block?
Mortar is estimated from the number of units you are laying, not from wall area, because the joint does the work: a standard brick has far more joint per cubic inch of wall than a concrete block does. The usual rule of thumb is that one 80 lb bag of pre-blended mortar mix lays roughly 35 standard bricks or about 13 concrete blocks with normal 3/8 inch joints. Change the units-per-bag figure above if your joints are wider, if you are laying oversized or thin units, or if the bag on your pallet quotes a different coverage.
There are two ways to buy mortar and they are easy to confuse. Pre-blended mortar mix already contains cement, lime and sand — you add water only, and the sand figure above does not apply. Masonry cement (Type N, S or M) is the binder alone and gets mixed on site with mason's sand, commonly around one part cement to three parts sand, which works out to roughly 2.5 to 3 cubic feet of sand per bag. Mixing your own is cheaper on a large job but needs a mixer and consistent proportions; pre-blended is far more forgiving for a small wall or repointing. Either way, mix only what you can lay in about 90 minutes, keep the board covered on hot days, and never re-temper mortar that has already started to set.
Coverage figures are rules of thumb and vary with unit size, joint thickness and the mason's technique. Check the coverage printed on the bag you actually buy.