What is the maximum number of guards that can be assigned to the vault door room fully upgraded?

While trying to decide which defense strategy is best to guard my vault against raiders, I came across some conflicting information about the room occupancy.
Some say that the only benefit we get from upgrading the vault door is that it gives us more time before raiders enter the vault (so I assume it will still hold only 2 guards), while James says in this great answer about room layout strategy) that you can place four guards at the gate.
Do NOT use the gate room for defense. You can only put four people in there.. its just not as effective as a full sized room can be.
So, what is the maximum number of guards that can be assigned to a fully upgraded vault door room? 2 or 4?
If it's 4, I'll fork over some caps to upgrade the room and have my guards there to minimize raiders time inside the vault, while also having a 6 people room next door armed to the teeth.
If it's 2, I'll save some caps and won't bother upgrading the room. I'll leave the guards in the room next door.
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How many people can guard the vault door?
The Vault door can hold up to two dwellers as an initial defense against raiders, feral ghouls or deathclaws.How many dwellers can work in a room?
Generally, each room can facilitate two dwellers. Merged rooms may be occupied by up to 6 dwellers. Production rooms require a specific S.P.E.C.I.A.L trait to run, while training rooms increase a specific trait of the selected dwellers.Can you merge upgraded rooms fallout shelter?
In fallout shelter, on android, one of the objectives that may come up is to completely merge four rooms. However, as stated in this question, you can only merge a maximum of three rooms together....Can pets have babies in Fallout Shelter?BreedBoxerLegendary Bonus+36-45%3 more rowsHow big is the vault in fallout shelter?
Layout. The Vault is built on a 20x25 grid, with one ground level and 24 subterranean levels below, each subterranean level contains 20 available spaces. Most rooms can built next to a horizontally adjacent room and take 2 spaces.FALLOUT SHELTER MAXED OUT VAULT! MAX EVERYTHING! 200 Dwellers (BEST LAYOUT!)
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