Do a 'search' for threads, in this section of BYC and also in the 'breeding, showing and genetics' section of BYC, for threads about inbreeding, sibling mating, parent-offspring mating, etc. You will find much valuable information.
Bottom line: what you get from a hatchery is probably not all THAT closely related to begin with. That said, the results of inbreeding (mainly when continued for a number of generations, not just once) are quite variable because they depend on three important things: how many deleterious recessive alleles are lurking in the gene pool you started with, how carefully you cull each generation, and how many chickens we're talking about here.
If there are a bunch of bad traits lurking in your starting stock, inbreeding can show problems within a generation or two; more often, you can go a handful of generations and then start seeing a decrease in hatchability/survivability; best-case scenario, if you start with chickens from a line that has been very carefully tended for quite a long time, there may not be a whole lot in the way of deleterious recessives in there and you may be able to go quite a while without any visible problems from (intelligently-guided) inbreeding/linebreeding.
And of course, no matter what you started with, you have to remove 'suspicious' or 'poor' individuals from the gene pool -- potentially also their offspring or even sibs, in some cases, if the trait does not show up til after maturity -- in order to keep things as tidy and on-track as possible.
The total number of chickens (kept over winter and used as breeders for the next year) matters because the fewer you hold over for breeding, the less diversity of genetics is going into the next generation. Continued inbreeding/linebreeding with a flock of 50 breeders will give you much more stable and acceptable results than continued inbreeding/linebreeding with a flock of 5.
I would suggest mainly not worrying about it until and unless you start to see problems occurring (which is often not so much "three headed chicks" type stuff as simply poor hatchability of eggs, or weak chicks); and at that point inject some new blood into the line. For a serious selection program it matters where the new blood is from; for just 'I want to have a nice flock of chickens' it can be whatever is convenient
GOod lcuk, have fun,
Pat