I was thinking about something like having an immovable rod stopping a door, so I think "what would my players do?"
The answer is they'll do all they can to move it. If they try to pull it by the description they have to succeed on a Strength check with a DC 30. If there's only 2 the Help action will grant them advantage, but what if more than two try to pull it all at the same time?
PHB 175: (emphasis added)
Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters.
The text says "two or more," and consistently refers to more than one helper. Thus, the advantage mechanic takes multiple helpers into account, and having two helpers (three total) is mechanically no different from having one.
in the rules, players can only benefit from advantage once, but as the dm you could reduce the DC the more people there are trying to open the door.
The PHB gives us an option outside of the Help action to adjudicate scenarios where more than two people are trying it.
Group Checks, PHB 175
When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check.
Ask everyone to make the ability check. If at least half the group succeeds, the entire group succeeds as a team. Otherwise, they fail as a team.
Note there is no critical success or failure for ability checks, so succeeding by rolling a natural 20 counts as only one success (or it might still fail if the modifier isn't high enough to reach the DC).
Yes, there is RAW on PHB 175 about help.
However, for situations outside the norm, you can decide with general rules and common sense whether it's a 1 or 1.5 person job, or if you can actually get everyone involved.
DMG page 5:
The rules don't account for every possible situation that might arise during a typical D&D session.
DMG Page 246:
When characters need to saw through ropes, shatter a window, or smash a vampire's coffin, the only hard and fast rule is this: given enough time and the right tools, characters can destroy any destructible object. Use common sense when determining a character's success at damagin an object.
When you need to reference "common sense" things, you can reward players for using real-world physics such as a lever to multiply effectiveness, or use the lifting/carrying rules:
PHB page 176:
Your carrying capacity is your strength score multiplied by 15
You can push, drag, or lift a weight in pounds up to twice your carrying capacity (30 times strength score)
Larger creatures can bear more weight, whereas tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag or lift. For a tiny creature, halve these weights.
So it will really depend on the given task.
For the example of the immovable rod: So here is a link to a lever force calculator: http://www.engineersedge.com/calculators/levers/page_levers_1.htm
Basically, if the target amount of force is 8000 pounds (W in this link), and we were able to get a lever underneath the object and W is length (x) 50 inches from a fulcrum, (L) length from fulcrum is where the player/party must exert force. If they can exert 500 pounds of force, they will "break" the rod's 8000 pound limit. So with a fulcrum object that won't be crushed under the load, and a lever object that also won't break under the load, a lever and fulcrum (proper tools) could make this pretty trivial if you have room to maneuver a lever/fulcrum in such a way as to get appropriate force.
