It is better to get all at the same time because integrating new chicks or birds into an older flock means a lot of fighting , pecking, bullying and sometimes much worse. If they grow up together it is lessenes considerably. Also if you only buy 2-3 and one dies, you are stuck trying to find a bird about the same age and size to be a replacement.
Rule of thumb however is 4 sq.feet per bird inside coop and 10 sq.feet per bird in outside run. If birds are too crowded they also fight, feather peck etc. etc. especially if they are kept inside for a lengthy period- for instance during horrendous weather.
Bigger is better because you will get addicted to chickens very fast and want every new breed you can find. Rather than make coop 2 and 3 and so on. Just shoot for the moon now.