In Rogue One we see
Galen Erso and his team of engineers,
which were at most a dozen. How could a dozen engineers have accomplished anything? I would guess at least a hundred engineers involved in an aircraft carrier...
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You are right that over a hundred engineers are necessary to do something big, but in this case you overlooked something. Galen was NOT responsible for creating the Death Star. He was ONLY responsible for creating a small part of it.
He was a Kyber crystal researcher. This means that he and his team were not responsible for designing the air conduct shafts. Instead, he was responsible for something else, namely, energy creation. As he said himself, the reactor is unstable - which lead to him being responsible for the energy creation of the Death Star. And THAT could have easily been accomplished by a relatively small team.
Especially one thing needs to be taken into account here. He didn't design the entry to the reactor or a specific shaft that was used later on to send a few small protonic things into the reactor chamber ;) . He said himself that he was sure that with the plans, the rebels could find SOME WAY to hit the reactor. This means he himself was not sure HOW. Therefore, we know that HE did not place that one shaft. The rebels just found it and used it alongside the information he could give - that the reactor is unstable and a single hit would blow up the whole station.
Thus all in all together: He and his team were just the designers of the reactor and also of how the Kyber crystals and the reactor worked together (the Kyber crystals MUST be somewhere in this as he was famous for Kyber crystal research). Everything else was done by other teams - probably similar in size each to his, or even bigger.
Thus, the small size of his team was sufficient for such a "small" task.