maximum engineer on a factory or build a structure

Is there a limit of number of engineer on a factory ?
I have the feeling that adding more and more engineer is sometime useless. Also if there is no limit the would be no need to build several factory ?

Same question When you build a structure... Maybe it is useless to have more than X engineer ?

There is no limit.
Building anything costs a certain amount of buildpower ("build time") to build 100%, every building has a certain build power ("build rate", lets call that number BR), engineers have certain buildpower (lets call it ER). So if you have enough engineers assisting so that ER = BR, your unit will be built twice as fast.

Time for unit to move out of the way for next unit does not change. Some buildings have such a high build power, for example nukes launchers (and the nuke itself has a high build power cost), that adding enginners does not do much.

Example 1:
T1 Land factory has 20 BR.
T1 Engy has 5 ER.
-> You need 4 assisting T1 engies to build twice as fast
-> 4 T1 engys cost 208 mass vs additional factory cost 240 mass -> engys more effective for raw buildpower

Example 2:
Nuke Launcher has 1500 BR.
T3 Engy has 30 ER.
-> You need 50 assisting T3 engies to build nuke twice as fast
-> 50 T3 engies cost 15600 mass vs additional nuke launcher cost 15000 mass -> building more nukes launchers more effective on average

You have to factor in roll-off time for factories. They spend about 13 seconds building a t1 tank and 2 seconds for it to exit. If you have 8 engineers assisting, it would take closer to 4 seconds to build, but still 2 seconds to roll off. The difference between build time and roll-off time means there are diminishing returns for adding engineers. If you have enough hives, the roll-off time will be bigger than the time it takes to make the unit.

Also, more engineers = more juicy a target for t1 bombers or other kinds of raids. So there is increased risk compared to having multiple factories. And engineers take time to build. If you want to increase your production as fast as possible, it's better to have engineers make factories, rather than using the engineers to assist factories.

For structures: you can run into problems similar to the "roll off time" issue for factories, when you are making multiple structures, because engineers have to move around and there can be traffic jam. If you have 10 engineers building 1 line of t1 pgens, probably half their time is spent moving rather than building.

From an efficiency perspective, you also have to look at the fact that a unit or building that is half-done can't do anything for you. If you have a T2 HQ and two T2 support factories, and each one has 1/3 of a Pillar, you have zero tanks you can use. If you have only a T2 HQ, and it's assisted by 15 engineers, in the same amount of time you could make a pillar. This is more of an issue with big expensive things like T2 navy.

Yes. More numbers:

A T1 land factory has a build power cost of 300. The equivalent 4 T1 engineers have a combined build power cost of 260x4 = 960.
That means its takes 3.2 times more buildpower (or time) to build 4 engies instead of 1 factory. In practice even more because of roll off time.

@Katharsas thanks so for mass T1, it is useless to have several factories with each 3 or 4 engineers. It is better to never have engineer on factories and build several ones instead. Good.
Maybe it is not the case for t2 and t3 because of the cost of t3 factories

To build quicker t3 units, instead of adding X t3 engineers (10 ? 20 ? 30 ?) To build twice quicker maybe it is cost effective and speed effective to build building that replace engineer (forget the name)

You can opare the stats here btw:
"time" = build power cost