Let's see... There's cost, hydraulic capacity, need, field big enough, ect.
Deere, among others sells planter units that can be mounted on any toolbar. There are quite a few toolbars available wide enough to handle as many as 48 rows. (biggest I've seen. In a picture. May be bigger) A planter that big needs multiple hydraulic hook-ups, electrical power supplied by the tractor, and a tractor with enough guts to drag something that big. Not to mention, a field big enough to turn it around.
Most likely, with the needs of a typical TBN'er, 2, 4, maybe even 6 row would be all they'd ever need. (maybe more) There are a few guys here still involved in production agriculture, but most are planting food plots or big gardens.
With a row unit like a #71 Deere, and a smaller tractor, you need to concern yourself with lift capacity and over-all weight carrying stability. Almost any tractor would still handle 4 rows, but by the time you work up to 6 rows, you have a wider and heavier planter that'll whip some tractors. Horsepower generally isn't too much of an issue with 2 or 4 row planters. By 8 to 12 rows,you'll likely be using a drawn planter as opposed to mounted. Hydraulic demends usually exceed what a lower hp tractor would have to offer before running out of horsepower. Also, as the planter gets wider, it gets heavier. It takes traction AND power to move weight. Even with 4wd, bigger planters usually take a heavier tractor to pull in all conditions.