I want to add new UDIDs in my developer.apple.com, I disabled old ones but still I am unable to add new UDIDs, we deliver our different environment builds to different clients so we need to add new UDIDs.

From the docs: https://developer.apple.com/support/account/

"Resetting Your Device List

If you are the Team Agent for your Apple Developer Program team, you can reset your list of development devices each year using Certificates, Identifiers & Profiles. You can register up to 100 of each device type for testing and Ad Hoc distribution per membership year. At the start of your new membership year, you and any Admins on your team will be presented with the option to remove listed devices and restore the available device count to 100 when you first sign in to Certificates, Identifiers & Profiles.

Although you may remove a device from your account during the year, it will continue to count against your device limit. When resetting your list, make sure to remove all devices you no longer use for development before adding any new devices."

>we deliver our different environment builds to different clients

For testing, right? You might want to suggest they reduce the numbers of individual devices and instead use just one or two that are shared during the testing phase. Or better yet, get them on TestFlight as external testers, where you can invite up to 10,000 and you don't have to add devices to your device list.