The webpage said that the incubator is for shoebox type boxes. Judging for the measurements and the two rack level, I think it could fit 4 shoebox boxes. In the shoebox you could arrange a clutch of 5 or 6 eggs. I hope this could help. The incubator looks very well made.
You can fit three clutches on each shelf. Mine has three shelves in the incubator to hold tubs. I bought it used and I assume that it was customized since their is an additional shelf between the two. Because of that, mine can hold 9-12 clutches depending if I put any on the floor of the incubator. With two shelves, it could hold 6-9 cluthces (depending on if the floor was used), but if you stacked tubs on the middle shelf it could hold 9-12 clutches.