In my opinion its much better to have a team working on a project than a single person.
Even if this means that your customer might have to wait a bit longer for his project to start (because other projects also occupy more people) everybody benefits because of increased productivity, better code and happy team members.
Here are my top five reasons why you should not leave one guy alone with a IT project...
Avoid Single Points of Failure
People get sick, are on vacation or might even resign from their job. You have to be able to compensate this by having other members that don't need weeks or months to understand the projects requirements or codebase. Avoiding single points of failures saves you from having to get new (other) people up-to-date which will cost you time, money and probably even upsets your customer.
Additionally your customer might ask for enhancements, bugfixes or even new (related) applications any time after the original project is finished. People that were involved on that project might be working on all kind of other projects then. If you have more than one guy that knows the domain and the code then you gain alot of flexibility in resource management.
Think Twice / Triply / ...
Another big benefit is, that team members have somebody to discuss any tasks with. These discussions might be about how to design a special feature or how the customers domain is modeled best. The members can save each other from producing bugs by reviewing each others code. They can also use Pairprogramming for tricky parts of the application.
Each member of your team also brings his special and individual skills and expirience. One guy might be better when it comes down to software architecture, another one might be the best choice to communicate with the customer and a third might be an expert at designing user interfaces.
Since IT-Projects require alot of different skills each of them will benefit from an increased bandwidth of skills.
IT-Projects can be frustrating sometimes. One person that works alone gets demotivated easily because he feels left alone with whatever is frustrating him.
Being able to talk about problems and motivating each other helps to stay in a good temper and thus be more productive. Having a good team and fun at work helps to endure frustrating parts of a project.
If someone has to accomplish everything by himself he might also get easy distracted. He might start browsing the web or he pays more attention his colleagues projects than to his own. But he will probably stay focused if you have a team that works with him, because he has someone to justify himself to.
A small daily standup-meeting (e.g. a daily SCRUM) where everyone explains what he has done the last day can help the team to stay focused.
Imagine how guilty you'd feel if everyone worked hard and you only watched videos on youtube instead of writing a unit test for the feature you implemented the day before.