If you have a second rank with halberds (reach) 24 people can attack a single character.
Adjusting the guards weaponry
My response to both of these would be to state that either situation seems HIGHLY unrealistic. Imagine what dozens of attackers running past your PC taking a swing each round would imply. This kind of 'Conga Line' would require all 200 opponents to enact a level of choreography never achieved in the real world! It is simply ridiculous. I mean, OK, if they are all controlled by a single mind flayer or something, maybe...
Likewise being pig stuck by 24 opponents, while it might technically be feasible under the rules (honestly, since 5e doesn't define any sort of 'grid' I'm not sure how you come to this conclusion to start with) but it SEEMS pretty unrealistic to me. My understanding of 5e's rules is that they don't have hard and fast RULES for stuff like this, and that the GM should be looking at the situation and determining what actually makes sense.
That being the case, 200 guards does sound like a PITA. OTOH its another of those tactical situations. If it is the right situation, or the PCs can recontextualize the conflict and put it in their own terms, then they should win easily.
Try this. "The horde of guards charges at your group of PCs. The wizard's lightning bolt kills a dozen or more, but there are just so many. As they reach you (the fighter and cleric front line,) each man jabs with his spear as he rushes past you to threaten your back line (rogue and wizard.) The first past you falls to your AoO, as does another to the cleric, but by now they are all around you. Jostling you. Attacking from all sides (with advantage because I'm a bad DM that likes optional rules.) You are struck repeatedly, no one wound being serious, but before you can retaliate you have enough wounds to be seriously concerned. You feel Bless end as the cleric stumbles under the many blows. The wizard is surrounded, and the rogue is gone. You aren't sure if she fell, or disappeared into the crowd. The wizard opens a dimension door and steps through, appearing far from the fighting. He pulls some bat guano from his pouch and looks back at you with determination..."
You make your 2 attacks, action surge for a couple more, and never get another turn.
Edit: replacing poo w/ guano