i found that occasionally a nipple would get a little stopped up. admittedly, i'm adding ACV and i think it's little chunks of the mother that are doing this. for this reason alone, i would add an alternate.
i had 7 chicks on 2 nipples and now that they are outside and using their big girl nipple watererer i've added a 3rd one. again, this is for redundancy.
from watching them it seems to me that around 3 chicks over 2 weeks old can drink off one nipple at the same time. so in theory one nipple should do you as they don't all have to drink at the same time.