I don't know of any official guideline for where or what-order you place the MBs on the sash.
I've seen many boys place them in rows in the order in which they were earned or received.
I've seen some put all the requireds in the center column so there are 12-15 down the center column with silver borders, and the green border ones on the sides and below.
I've even seen some start part way down in the front so they can put all the requireds at the top, and the green-border ones below. Then the cluster of badges grows in both directions as they earn them.
However you have chosen to put the MBs on the sash, you can only fit so many on the front. I believe the sash comes in 2 sizes. The bigger one has enough room to fit 60 MBs on the front (from the shoulder to the very bottom.
My son is not big enough to wear the longer sash unless he wants it to hang to his knees. I made a fold in it, and sewed it so that the extra is in the back and can be let out as he grows and needs more room. I haven't gotten to that point yet, but will soon need to start putting MBs on the back, or let out the fold to make room for them above the ones that are currently showing on the front. His goal is to earn all 121 MBs, so we're planning to have 60 on the front, 60 on the back, and the last one will have to sqeeze in on the bottom point (half on the back and half on the front).
I've never seen it but I've heard of others doing it this way.
I've also heard of a boy who earned all the MBs, but wanted to display them all on the front, so he sewed 3 sashes together and made one wide enough to show them all on the front. I can imagine that this was a bit cumbersome to wear and looked a bit odd, but he apparently liked it.
Eagle Scout 1987
OA Vigil Honor 1986
Fox - WE7-590-05-2
Currently - Troop/Team/Crew Advancement Chair & Dist Webmaster
Previously - SM, MC, CM, ACM, ADL, ASM, COR, Dist Camp Chair, PL, SPL, Scribe, Songleader, JASM, OA Chapter Officer, ...