This can be done by passing the tagLimit config through to TagMultiselectWidget.
|Allow HTMLUsersMultiselectField to limit number of selected items||mediawiki/core||master||+9 -0|
|Open||None||T139810 RFC: Overhaul the CheckUser extension|
|Open||Niharika||T236225 [Epic] CheckUser 2.0 Improvements|
|Open||Niharika||T237039 [Epic] CheckUser 2.0: Preliminary check|
|Resolved||Tchanders||T237299 CU 2.0: Sticky highlighting in Preliminary check and Compare tabs|
|Open||Niharika||T237593 [Epic] CheckUser 2.0: Compare|
|Resolved||Tchanders||T237300 CU 2.0: Sorting in the Compare tab|
|Resolved||Tchanders||T237298 CheckUser 2.0: Display the data fetched on special page (preliminary check)|
|Open||None||T238603 Placeholder text on Special:Investigate seems unnecessary|
|Resolved||dmaza||T242945 CheckUser 2.0: Provide a way to start a new investigation [xsmall]|
|Resolved||Tchanders||T237034 CheckUser 2.0: Input form|
|Resolved||Tchanders||T238240 Allow HTMLUsersMultiselectField to set max limit on number of selected items|
I don't see anything currently that uses HTMLUsersMultiselectField at the moment. I don't think there is anything for me to test until other CheckUser work is complete.
I am going to mark this as done. Testing will be done as part of later work.
I believe this is used on https://test.wikipedia.org/wiki/Special:Preferences in the "Prohibit these users from emailing me" field as well as on https://test.wikipedia.org/wiki/Special:Preferences#mw-prefsection-echo in the "Do not display notifications from these users" field. I believe these fields are enabled through config (I can figure out the configs if you would like).
After applying patch in https://gerrit.wikimedia.org/r/552269, I am now limited to entering 2 users/IPs in the Special:Investigate form. (While writing this thegoodplace doesn't have that patch, so form does not quite behave as it should.)
I briefly retested the "Prohibit these users from emailing me" input in Special:Preferences. In particular, I tested adding lots of usernames to the input just in case we have accidently introduced some sort of default max limit. I could enter over 500 usernames.