I cannot open a new tab in safari

Ensure that you have updated to the most release of iPadOS - currently iPadOS 15.5.

If you have already updated, perform a forced-restart of your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

How many Safari tabs are currently open?

There is a fixed limit to the number of open Safari tabs - after which the “+” button will not be available.