How many devices can I register with my iOS Developer Program Account?

You can test with up to 100 devices per year.
This is to prevent developers from selling their apps outside the app store by using the developer certificate and registering an endless number of devices.

Even if you remove a device from the list, it will still count towards your 100 devices per year limit.

You can use up to 100 devices per year.

Per Apple Developer Account Management :

You are allowed to register up to 100 iOS devices for testing and Ad Hoc distribution per membership year. Please Note: Although you may remove a device from your account, it will continue to count against your 100 device limit.