根据苹果，你可以安装最多10台设备。我认为也许他在同一台设备上安装了两次该应用程序

Apple clearly mentioned on their page here

"Each app can be tested by up to 25 members of your team who have been assigned the Developer or Admin role in iTunes Connect. Each member can test on up to 10 devices."

But surprisingly one of our internal testers invited to test 2 apps. He installed the First app in 3 devices.

But the second app he is able to install in 11 devices.

Can anyone please tell me what exactly limits and constraints of test flight internal tester?

Here is the Tester Profile:

You're right, according to Apple, you can install in up to 10 devices. I think that maybe he installed the app twice in the same device