Thumb rules are 1 box per 4 or 5 laying hens, plus one. With 26 hens, I'd call that six boxes or seven if you have production breeds or decent to good dual purpose. If your breeds are infrequent layer types, you don't need as many boxes. You can use fake eggs, golf balls, and similar to give them the idea of where to lay, don't be surprised if the first few eggs don't actually appear there. New layers tend to miss a few times, even when an experienced flock is around to demonstrate proper behavior.
Makes sure they are dry, well padded (straw, pine/whitewood shavings, etc), and below the level of the roosts (to discourage birds from sleeping in there.) Also, don't be surprised if you get more eggs in one box than another, if they seem to have favorites, or "wait in line" even when other boxes are available. Neither should you be surprised when favorites change. Those are all normal chicken behaviors).