The smallest and adequate amount of chicks to add to an already-existing flock, in my opinion, would be about three or four. The important thing to remember is that the chicks should only be added to the flock when they are around twelve weeks old. They can get integrated at six weeks, but I would not want to stress them too much.What in your opinion is the smallest amount of birds you should add to an existing flock at one time, @SwampPrincessChick? I was considering adding 3 at a time every few years to keep the egg production at a level my family can consume, until I reach about a dozen birds. Is that going to be too difficult on the new chicks and the flock? The greater good is my birds’ well-being, rather than wasting eggs. Should I just buy 8 more chicks in one go to spare my current 4 too much stress? If so, how soon should I do so? My girls are only 2 months old.
Consistenly adding birds to your flock will stress the chickens out. The pecking order will have to be changed too many times if you do that, and that is not good on your flock's health. I would suggest you to buy the chicks all at one time, so they can grow up together and have less of a stressful life.
As for how soon you should purchase your new chicks, are you in need of more chickens right now? I would suggest buying them next year, since it is getting too late in the season for more chicks. If it is not getting cold where you are located, you can go ahead and purchase chicks if they come in soon.