I agree with Dinosaw, introducing a single hen is always a problem. You haven't had your birds for long, but they will have established their flock structure and will make a single newcomer feel permanently isolated and unwelcome, maybe beating her up and injuring her badly if you just put her straight in with them. If you have room for two more, and can divide your run with some mesh, and provide a temporary roosting place for the new birds for two weeks, they will have a much easier time of it when they join the others. In any case, you should always plan to separate new birds for this length of time, in case they are developing any illness as a result of the stress of being moved to a new environment. Ideally they should be in a pen away from the others for proper quarantine, but dividing the run is the best that many of us can manage, and need not be expensive. This time gives them a chance to settle in and bond as a pair, if you have two newbies, and this will make it more easily possible for them to integrate with the other hens as they will support each other within the group. Being able to separate hens from time to time, for various reasons, isn't really an optional extra but a necessity, eg when one is ill, or has been bullied and injured, as well as when new hens are introduced.
If you haven't got room for two more, then it might be best to keep the number you have got and see how you get on. Remember that each adult hen needs a minimum of 2 square metres of run space, more is better, so unless you can provide at least 12 square metres of run, it would be a good idea to hold off getting more birds. If and when you do get more, try to ensure they are at least as old as the present hens, as young birds not yet mature enough to lay will be unable to have the confidence to make their mark among older 'strangers.'