My nest boxes are stacked 3 over 3. The birds do prefer the upper boxes. General rule of thumb is 1 box per 4 birds. OP would do well with 2 boxes. Make sure they are big enough. Birds stand up when they lay an egg. Many folks don't take that fact into consideration and do not make the boxes tall enough. An other option would be a community box where many birds can lay eggs at the same time. I believer JT's roll out is a community box. My birds LOVE the larger boxes. I have a broody area that has 2 of these storage crates (I removed the top 2 slats from the front of each box) https://www.lowes.com/pd/STOR-Crate-12-5-in-W-x-9-5-in-H-x-17-5-in-D-Wood-Bin/50375206
plus some extra space, and they LOVE that area.
plus some extra space, and they LOVE that area.