1-2 (in extreme cases up to 4-5, if they really think alike) scientists are the brain behind a project, they'd have a small group of aides/lab technicians to help with the dirty work (but NOT thinking or taking credit), in some cases gruelling and less important work gets outsourced, or, if a project is very big and important, it's split into parts for several groups like that to work simultaneously (ex. Manhattan Project), but in such a case, a lot of diplomacy is needed to appease scientists' egos and curb the competition between groups (no such big projects in XCom it seems).
But it's always the quality, not quantity of that small group that matters, groups more numerous than 2 scientists are prone to intense bickering and inefficiency, still, giving them more lab technicians and money would help ofc.