dnd 5e – Is there a limit to how many allies can use the help action, to help you in combat?

With suitable timing, each help action grants advantage on one attack

As you’ve surmised, for this scheme to work all the helping creatures except the first one must ready actions with triggers of “after my ally’s first attack”, “after my ally’s 2nd attack” and so on, because if they all took the help action before your turn, then all the extra help would stack up on your first attack and be wasted, since multiple sources of advantage don’t stack. However, assuming you can organize all your helpers appropriately, there’s no reason that each help action can’t grant advantage on one attack, giving you advantage on as many attacks each round as you have helpers. Obviously at some point you could run into other limits such as the number of helpers able to get into melee range, but the vast majority of the time, the limiting factor will be the number of attacks you can make in one round.

Each help action grants advantage on the next attack

I’m claiming that each help action grants advantage on the next attack, but the wording of the help action doesn’t fully spell this out, so let’s think about it a bit. The relevant part is:

If your ally attacks the target before your next turn, the first Attack roll is made with advantage.

Which attack roll is the “first” one? It’s a bit ambiguous. Clearly it’s not the first attack roll of the campaign, and it probably doesn’t mean the first attack made by the ally during the current combat. That would make the help action mostly useless after the first round. Hence, the “first attack roll” must be implicitly referring to the first attack roll within a certain time period. The end of that period is given explicitly: it’s the start of the next turn of the creature taking the help action. But when does it start? The only start point that makes any sense is when the help action is taken. Hence, if multiple help actions are taken at different times using readied actions, they can grant advantage to different attacks made by the same creature during a single turn.

In other words, the wording above is equivalent to (addition in bold):

If your ally attacks the target before your next turn, the first Attack roll after you take the help action is made with advantage.

Presumably, the text doesn’t specify this explicitly because usually it doesn’t matter. The only time it would be ambiguous is if a creature readies a help action to be taken during another creature’s turn, which is a rare occurrence. And it is well established that 5e doesn’t try to spell out rare interactions in the rules, but instead expects the DM to handle them.