Hi @JACK_LAI_1117 :
I think this problem should start from two aspects: data source construction and app construction.
I assume there are two events for the user to choose: "event1" and "event2".
- 1\Can a user select two events at the same time?
- 2\Can a user select the same event multiple times?
I assume that a user can only select one event at a time, and an event cannot be selected multiple times.
I'v made a test for your reference:
1\My data source(TheList-SharePointList):
Column (click to edit) Type
2\Add two buttons(Event1/Event2)
Event1
OnSelect
Patch(TheList,{Title:Text(Now()),DisplayName:User().FullName,Event:"Event1"})
DisplayMode
If(User().FullName in TheList.DisplayName || CountRows(Filter(TheList,Event="Event1"))>=20,Disabled,Edit)
Event2
OnSelect
Patch(TheList,{Title:Text(Now()),DisplayName:User().FullName,Event:"Event2"})
DisplayMode
If(User().FullName in TheList.DisplayName || CountRows(Filter(TheList,Event="Event2"))>=20,Disabled,Edit)

Best Regards,
Bof