"Too many tabs" error notification is customizable, badge can switch between different counting modes and it supports managed storage for rollouts via policy (Example file).
Unlike similar extensions it does not support per-window tab limits, only a global limit across all windows.
It is not very optimized as I lack experience outside of Excel-VBA, but it works and shouldn't slow down anything, at least it's not noticable on the old ~2009 dual-core Linux machines we use this on.
This extension allows the user to specify an upper limit for tabs open at the same time.
It displays the number of currently opened tabs.