well, whether 5 fit along a dimension is just easy division. gardens trays vary slightly in this respect, and more so if diy. the cubes come in a specific size, though.
unless your light is perfectly sqare in shape and dsitribution of diodes, it's probably spreading out a bit wider on one axes than the other. Even fairly square lights utuilizing led strips/bars. if you draw lings to a point out parallel perpendicular to bars (in center or wherevere as long as apples to appels) you'll wuickly see how many more are much closer going out perpendicular from bars, you might get 4,5' x 5' with a strong 650-720w sized light. Or 150% more sq ft if autoflowers on 18/6. So, it's possible going a bit further out on one end can work well. consider umol/s of yor light, the shape and stype of frame.. guesstimate the shape it makes and stick to those proportions as best you can. that'll "center" your best canopy coverage relative to strongest distribution of light.
how you fill that space will mostly impact necessary vege times for your photperiods. if you have all the maintenance stuff automated, more plants will get things done faster, up to a point. how much more work does that add for you? you look at the positives and the costs relative to your context.. too much info is not available to be specific.. can't read know your personal preferences etc.