Get count per timestamp for each ID

Yes datanau001 - one of the few times a column is better than a slicer. Slight tweak to my measure:

Item Count = 
VAR varCurrentValue = 'Table'[Item]
VAR Result = 
    CALCULATE(
        COUNTROWS('Table'),
        'Table'[Item] = varCurrentValue,
        REMOVEFILTERS('Table')
    )
RETURN
    COALESCE(Result,0)

Strictly speaking the COALESCE() at the end isn't necessary as you'd never have a blank like this in a column, but I am just in the habit of using it with any measure that has COUNTROWS() so I get a true 0 vs <BLANK>

You could just use "Result" at the end after the RETURN statement.