you're asking good questions. if you want them to retire and not be eaten, you have to plan carefully and as another poster said, have some restraint when adding to your flock (unlike me). unless perhaps if you are extremely lucky and not limited by either budget or space.
i grew up raising chickens in the country but have been raising chickens in the city for over 10 years. we are very space constrained. we'd ideally like to get 2-4 eggs per day. it's been hard to balance our egg needs with available space and the chickens' various life spans. i caused the biggest problem when i last added to the flock ~2.5 years ago. chicken math set in at the feed store. i wanted to add 2 pullets to the flock but figured one c.hick might die, and one might turn out to be a rooster... so I got 4. Now I have 4 pullets plus the two existing that are now 5 and 6 years old. Technically I could squeeze up to 10 in my run but don't really want them in such tight conditions for a variety of reasons.
in hindsight, i would have looked for a good breeder and bought 2 full grown ~ 6month old pullets instead of 4 chicks. then i'd know they are healthy, female, and past the infant mortality phase. i'd also rather buy from a breeder than the feedstore hatchery chicks.
what i would plan on doing if your goals are like mine is to start with the bare minimum # that will give you enough egges (i'd start with 3 minimum - then you can lose one without NEEDING to add another), and then plan on adding 2 every 2-3 years. always add in pairs as it is too hard on the loner if you just add a single hen. Hard to say how many you will end up with as their life spans really seem to vary - I've seen most live 6-10 years, but there is the occasional chicken that mysteriously dies younger and there are reports of much older. doing this, you may run out of space or budget at some point. i hope to be in the country by then. consider the breed carefully - you want them to be good layers (i assume), docile, tolerate confinement. if you get chicks you might want to consider sex links unless you are okay with the 1 in 10 or 20 chance that one of them will turn out to be a rooster. i doubt they are legal where you are, and they are not easy to find homes for unless you're okay with them being eaten.
as for introducing new pullets, this is also a wise question. success will vary but in my experience it has always been a royal pain in the butt. lots of fighting. it is stressful to me and the chickens but it eventually works out. you also risk the introduction of disease, in fact conventional wisdom is to replace the flock completely every few years to reduce the spread of disease. if you're building or adding on to a coop it is best to design it with partitions in mind for the introduction period (separate but visible through a fence).
BTW my 5 year old silkie is laying 2-3 little eggs per week, even in winter.