I try to have at least one researcher in each field.
At least 25% (up to 50%) of labs are allocated to the best construction/production researcher, increasing research rate all the time --> compound interest
Early to early-mid game, 10 labs for important tasks, the rest splitted between the other researchers.
When I get more labs, I try to have two researcher per field, so there is a backup, if the main guy bites the dust
This means I have usually quite a few projects running, though half of them are normally only 1-lab-jobs.
Of course, if something is needed very urgently, as many labs as possible are lumped together for the task