we will have another 100 plus devices coming on campus and currently have 5 AP 121s deployed across the campus. how many simultaneous connections can the AP 121 support? if we use a few new AP 141s, how many simultaneous connections can they support?
Since you are an existing Aerohive customer, I would suggest you use the planning tool in the hive
import your map, build your walls, doors, and other building obstacles. figure out your requirements like -70 or -67 etc.. Place the model of AP on the maps to give you an idea of coverage and capacity. Do not plan at full power and if plan on disabling b and other data rates it will decrease coverage.
Are you looking at coverage or capacity
Data only network
what sort of devices? Know what devices you currently have and their capabilities and know what is expected to come on campus.
Types of applications to be used
I am assuming this is a school
and assuming you are going 1 to 1
I would recommend 1 AP per classroom which would be about 22 to 25 "1 to 1" users
but you can certainly load balance the devices 25 on 2.4Ghz band and 25 to 30 on the 5Ghz band.
but really it has to be tuned to your use case and your environment
I would recommend reading
CWNA: Certified Wireless Network Administrator Official Study Guide: Exam PW0-105, 3rd Edition