Max. Regiment size (model count)

The reasoning has to do with Game Balancing.Check the thread on "Triple units" in this section. Baradaeg has answered to this.

Tbh, large Blocks of Infantry(>20 models/unit) were actually only a thing in WHFB 8th. Edition. In all prior editions, we played , even with skaven, skellies or goblins, 20-30 max in one block....And even in 8th. mainly for lower tier infantery units such as States troops, Skaven-slaves ( I had 3 blocks of 60 in my Skaventide, O&G and to some extend, Beastmen ). High tier infantry like WoC or Bi-Handers came in blocks uf 18-24 max ( around 450- 500 pts each including banners !). Except for a few deathstars which were banned by most comp-rules anyways.

Elite infantry came in blocks of 5 -10..in WHFB as in AoFR. The triggers for the huge blocks in 8th. were mainly the ominous new "steadfast" and "Hordes" rules .and they were most of the time not enjoyable.

Lots of money, lots of model assembly andd painting time, little effect in the game other then delaying the inevitabLE. And , due to bad game design,often misused. To counter hordes, GW invented a crazy magic system with some spells removing entire units from the gametable..ideally in turn 1 or 2. Normal weapons were not anymore able to deal with a 40 Model steadfast unit , buffed by a few characters...very lame and boaring....and causing unneccessary powercreep in an, otherwise enjoyable, game.

In addition, most WHFB games at the time were played with only rudimentary scenery and the obligatory hills for arty on both sides of the playing table, allowing some movement for such unwieldy unit-blocks.

Also WHFB lasted 6 rounds instead of 4 as with AoF/R...

Typically, AoF/R is ( like most OPR Games) an objective based game on tables with lot's of scenery. Which actually favours quick and agile units hitting hard to claim an objective and maybe keep it for round by "larger" and cheaper units rather than shovelling pretty clumsy blocks of, otherwise rather useless, chaff over the board.

I played a number of OPR games now ( always 2.500 pts ) and I used mainly armies which could be considered "Horde" ...many bodies. I found, on a typical OPR gaming table with 4-6 Objectives , various buildings/hills/forest/swamp etc, blocks of more than 20 models of infantery can become quite difficult to handle and actually offer no real advantage in a game where you start at latest in round 3 to think how to make up for missed VPs quickly taking over new objectives or fullfilling open missions...

Hope this helped a bit.

Glad I could help a little. When playing 6./7. Edition, our largest blocks in Elite are around 20 models of Infantery anything more was way to expensive ( in points ).

However, the game mechanics in AoF asks rather for more activations in a player turn then for larger units to activate.

However, As there are no restrictions on how many units you could take ( as long as you stay in the points limit you agreed upon ),, you can of course take 2 units of 20 and keep them in close proximity to each other in order to work together almostvas one. If you can keep them closely aside from each other and activate them one after another,  you could even use a kind of  "Detachment" tactics as the empire did in 6.Edition. 

Also, if really want to play such huge regiments, you can of course "houserule" the max. Size. and try to make the best out of it.

You have to see , that AoF not  a simple ( poor man's ) version of WHFB or WHFB "light". It is , in particular when you include just a few of the many special rules, really a different game on it's own for which you can use you existing collection of Minis. Bit like KoW and other miniature agnostic rules.

It does, however, provide for a lot of fun and that is, what players ultimately look for, I think.

All the best and have fun.