Usually, when there’s a timeline problem, the first instinct is to solve it by adding more people to the project.
The logic makes sense. One person takes four weeks, so two people should take two weeks. It is a reasonable assumption, and it is wrong often enough that it needs to be examined before anyone acts on it.
Frederick Brooks wrote about this in 1975. Adding people to a late project usually makes it later, not earlier. It has not stopped being true.
Here is what can happen
When new people join mid-project they need time to get up to speed. They need context, the history of decisions made, why certain things were done a specific way, the nuances of how the client communicates, and where every workstream currently is. A new person gathering that context means someone producing is now teaching instead of delivering.
A lot of project work is quite sequential. Some tasks cannot be split across ten people working at the same time. Someone has to go first, someone reviews, someone approves, and the next phase depends on that output. The structure of the work sets a ceiling on how much additional resources can speed things up.
None of this means adding people is never the right call but it should be done deliberately and early, not as a last-minute idea to salvage the timeline.