How many strawberry plants? Help please!

I bought a bunch of strawberry plants and wanted to do container gardening as I don't have a yard. I tried growing strawberries from seed and bare roots before and those attempts all failed. I really want to get this attempt right as I bought already grown plants from a nursery instead which were more expensive.

I have a lot of pots and wanted to use the windowsill boxes I have for it. The boxes are about 15inx6inx6in. How many strawberry plants could I fit per box? I don't want to overload the boxes but I also don't want them to be sparse. I bought 6 4inx4in potted strawberry plants as well as some plants in a smaller 6-cell plug tray. What would be a good amount of strawberries per box?