a value mapping could potentially be used in thousands of items. how would such cases be handled ? limited to 50 by default ?

The same way it is (not) done in 'CONFIGURATION OF MEDIA TYPES'

As mentioned before the list should be unique (E.g. --SELECT DISTINCT _key...")
In the first step the elements by value mapping should be limited by 'Max count of elements to show inside table cell' and in long term might get supplemented by ZBXNEXT-1796.

media types are different as there are way less actions than items

In my case both, actions for media types and items for value maps, will be by far more than 1k.
Therefore a limitation is welcomed for both

After cleaning up value maps, I have voted for this issue
A hackish way to find out how many items use a particular value map:

select count(*) from items where valuemapid=70;