IOS – how many devices can be registered for iOS Enterprise programme

A $99 "company" developer membership allows the administrator of the account to set up many "developer" accounts. You should get the administrator of that account (whoever set it up originally, most likely) to create accounts for each of your developers.

You might need to share the admin account for things like provisioning and app submission, but for most of your daily work, you can have your own login that shares the developer membership.

Each year when you renew your program membership you can remove devices added in previous years. So if you had 100 devices allocated, you could delete 5 and add 5 new devices.

As soon as you add a new device, you can no longer remove devices for the remainder of that year.

So what they are telling you is each year, after renewal, delete any devices you need to remove first. Once you add your first new device, you are only permitted to add devices until your next renewal up to a maximum of 100 devices.