It will depend on what you can get. You might get some point of lay hens, which would be a bit younger than your current hen, right? If so, I would get three birds that know each other. Because they are younger, and probably be a wee bit smaller than your current bird, they will kind of equal out.
One older bird +2, home turf advantage, +1 = -3 younger, smaller birds.
OR if you can get older birds, then I would get 2 head. 3 might give an advantage to the new birds, plus you whole flock is now older birds. It is better to have multiple ages in a flock, so that you are never completely starting over, you just add a fee, lose a few over the years.
Take a good look at your run, if the only place for your birds to stand is on the ground, you will have more trouble. Sticking in some roosting poles in the corner, putting a pallet up on blocks, can really add square footage to your set up, give space where birds get away from each other and out of sight.
Also, many will recommend quarantining, and while I am not in favor of getting sick birds, with just one hen at stake, I would not worry about it. Take a good look at the birds you are getting, check them over for lice or mites, look at there nose and eyes, and most likely they will be fine. DO NOT buy from an auction or a swap, those you really NEED to quarantine.
With just one bird, getting the flock going again is the most important.
Mrs K