But there are junior officers all over the place in red (TNG). They are obviously not making decisions and giving orders all day long. And there clearly don't need to be very many helmspeople on the ship.
First, are you sure all those people are officers? As I remember, the TNG uniforms didn't distinguish between officer and enlisted ranks all that well (unlike TOS). O'Brien wore the same style uniform as everyone else and had two pips on his collar, even after they started referring to him as "Chief".
However, it must be acknowledged that Starfleet is lousy with officers, way, way, way more than are necessary to manage any boat (inspiring a joke in ST:There Be Whales Here). Many of the tasks we see officers doing in Star Trek (like actually driving the boat) would be done by enlisted personnel in the Navy. Even a ship as big as the D shouldn't need more than a few dozen officers to manage1
So, this is speculation, not an authoritative answer, but I think it's reasonable: all those juniors are rotating through different departments for on-the-job training, and for administrative purposes they're all in the "command" division until they receive their permanent assignment (which may not even be on the Enterprise), at which point they wear the color of the division to which their department belongs.
To make this clearer, this is not an authoritative answer, just speculation going by what I understand about shipboard organization in the present-day US Navy:
Shipboard organization is broken down into multiple departments - Engineering, Medical, Deck (maintenance of non-engineering systems), Operations, Administration (even the 24th century needs paper pushers), Supply & Logistics, Weapons & Tactical, Communications, etc.;
Each department has a department head that's a senior officer (Lt Cdr, Lt), and they report directly to the CO (or XO - depends on circumstances, I think);
Each department head would have at least one assistant (Lt or Lt jg), plus maybe a couple of junior officer direct reports;
Rounding out the department would be chiefs, petty officers, and crew who do the actual work2.
Junior officers would rotate through various departments over time, picking up OJT by shadowing officers permanently assigned to that department. They would probably spend most of their time writing status reports and doing other administrative scut work. They may have a couple of enlisted direct reports that they get to boss around.
So, who are all the damned redshirts running around the D? I suspect the majority of them are enlisted (who work in the departments that would logically fall under Command like Navigation, Operations, etc.). Again, the TNG uniforms don't distinguish between officers and crew all that clearly. You'd have the Command department heads and their assistants.
The rest are a gaggle of ensigns and lieutenants jg who are picking up training in various departments, but for administrative purposes are assigned to Command (maybe so the XO can review their performance and decide on their permanent assignment).
Again, pure speculation, not based on canon.
1. When I take over the franchise, the majority of major characters will be chiefs and petty officers
2. All those times you see Scotty climbing around the Jeffries tube, or Spock rewiring his own console, or Kirk helping bring the Constellation back to life? Yeah, doesn't usually happen that way. Officers give orders; enlisted execute orders.