Mason bees need fresh housing each year.
There is no way around this rule, and its violation is the source of more trouble than any other rule. Your success with Mason bees comes with your grasp of this rule and your willingness to follow it. Find a system of housing which works for you and which you can abide by each year.

Mason bees need the right type of housing.
Mason bees require a physical structure for their larval cells - but they are dedicated versatile builders. Be sure the nest material is non-toxic. Treated lumber will poison the larval food. Plastic is “safe”, but doesn’t breath; bees also have a harder time making mud stick to plastic walls.
A nest tunnel of 5/16” inside–diameter is often suggested. But tunnels 3/8” diameter also work well: the bee needs more mud for each cell, but fits each tunnel with more cells and thus bees.

Mason bees choose nests based on nearby food.
If mason bees emerge from their winter slumber and find plenty of pollen and nectar within a couple hundred feet, they’ll stay put. But if better food is down the road they may shift operations. If nothing is blooming, you might loose your bees.

Mason bees need mud, nearby, to make nest cells.
Mason bees need a nearby source of mud (moist dirt). Some people make a mud-pit. If your soil is very sandy, add some soil with a higher clay content. The more convenient the mud, the easier / faster it is for the bee to build, as one cell can take 25 or more mud trips, (preceded by this many pollen trips), before the cell is completed.

Mason bees work in cool - but not cold - weather.
It is common for Mason bees to wait until 11:00 a.m. or noon before getting to work. The warmer they are, the sooner they get up. They can work in overcast, even drizzle, but not in rain and wind. In general they are active when it becomes 55° and warmer. If their housing faces southeast and warms up with morning sun, and is sheltered from the wind, they may start earlier. It is common for them to work until dusk. At night they usually tuck themselves into an open hole head first with their hind end almost sticking out.

Mason bees pollinate from March through May.
Mason bees are active adults only in the early spring. They are excellent pollinators of fruit trees and early flowering plants but by early June their adult reproductive phase has ended. Plants which need pollinating after mid or late May need help from other sources.

Mason bees are a solitary bee.
This is a solitary bee. Each female is tolerant of her neighbor, but not cooperating with her. While at the peak of activity the population may resemble a hive, it is not. It is an apartment and you are the landlord. Mason bees do not tend their progeny. They have no traffic cop at a hive entrance. They are not daughters of one queen. They work alone.

Mason bees can become too plentiful.
Mason bees are easy to raise - a good “starter” bee. When providing everything right, you can increase six-fold a small number of bees within a year. Don’t get overeager and raise more bees than an area will support or you can manage! Exceed the limit and stresses increase, mix-ups increase, pests increase. Too many bees at one time can strip the pollen resources, or just as likely result in more fruit on a tree than it can handle.

We can't fully say how many is too many. Mason bees are VERY efficient fruit tree pollinators. Research in orchards suggests that as few as two or three female mason bees can effectively pollinate an apple tree and 250 to 300 females can pollinate one acre of apples.