It will be drastically reduced if you have a true quarantine for a month. Which means completely separate by a fair distance. Merely putting a fence between the two flocks is not quarantine.
Also, it depends on the where you are buying them from. If a swap or a auction, I would be very careful. If you know and can visit the chickens in their present location, and all are healthy and active, not quite so critical, but still practical, much more critical if you have a very large flock which equals a large financial investment. If you have a large emotional investment in your original flock, then again the quarantine is a good idea.
The thing is a real quarantine can be difficult to do in a typical back yard, and a pretend one is not worth the hassle. So either you don't ever add any more birds, or add only just hatched chicks that you raise, or you risk it. I have often risked it, with no problems, but it only takes once.
MrsK