TopN Dynamic Measure

Hi emmawdmi22 

You need to have a separate disconnected table that contains the topn numbers to be used as a slices selection. 
Create the following measure, place it in the filter pane of the table visual and select "Is not blank" then apply the

Filter Measure =
VAR N =
    SELECTEDVALUE ( Selection[Number] )
VAR CurrentRoom =
    VALUES ( 'Table'[Room] )
VAR SelectedRooms =
    ALLSELECTED ( 'Table'[Room] )
VAR T1 =
    ADDCOLUMNS ( SelectedRooms, "@TimesUsed", [Times Used] )
VAR T2 =
    TOPN ( N, T1, [@TimesUsed] )
VAR T3 =
    SELECTCOLUMNS ( T2, "@Room", [Room] )
RETURN
    COUNTROWS ( INTERSECT ( CurrentRoom, T3 ) )

emmawdmi22 
Please try

Filter Measure =
VAR N =
    SELECTEDVALUE ( Selection[Number] )
VAR CurrentRoom =
    VALUES ( 'Table'[Room] )
VAR SelectedRooms =
    CALCULATETABLE ( VALUES ( 'Table'[Room] ), ALLSELECTED () )
VAR T1 =
    ADDCOLUMNS ( SelectedRooms, "@TimesUsed", [Times Used] )
VAR T2 =
    TOPN ( N, T1, [@TimesUsed] )
VAR T3 =
    SELECTCOLUMNS ( T2, "@Room", [Room] )
RETURN
    COUNTROWS ( INTERSECT ( CurrentRoom, T3 ) )