I just started the game and have therefore only one scientist. But for new research, I need at least two scientists or must outsource (costs more money). How do I get more scientists for research?

You have to use the world scan thingy and look for yellow targets. Then walk up to them with your weapon holstered and bribe them to join you. They are pretty scattered about the map iirc.

If you 'purchase information' for the the Black Market Cyber-clinic mission in the Wyvern compound in Downtown, and then get your Support agent inside that facility, that would also give you two new researchers for that early research. If you do that mission before purchasing information, or if you get those researchers killed during the escape, you lose the chance. Later in the game there are three different missions that can/will also give you new researchers.

But yeah, activating World Scan is generally the way to find more researchers.