The number of team members is irrelevant.
Whether or not to invest time in tool development is a simple cost/benefit analysis.
Make an estimate how many hours it will take to develop a tool and how many hours of work it will save you in the long run. When the first number is smaller than the second, go for it, otherwise do it manually.
Also, never forget to check which 3rd party tools are available for your purpose. Adapting an existing tool is usually magnitudes less work than developing a comparable tool from scratch, even when it means that you might have to do some modifications to your engine, the tool itself or write some form of middleware. The first 3rd party tool you should consider using when planning a game development project is the game engine. Very few game projects have so unique and exotic technical requirements that using a standard game engine is more work than developing your own from scratch.