count rows with some conditions

Try

Num occurrences = 
SUMX(
ADDCOLUMNS(
SUMMARIZE( 'Table', 'Table'[idclient], 'Table'[idticket] ),
"num instances",
var firstInstance = CALCULATE( MIN( 'Table'[Date_time] ), REMOVEFILTERS( 'Date' ) )
RETURN CALCULATE( COUNTROWS( SUMMARIZE( 
FILTER('Table', 'Table'[Date_time] > firstInstance ),
'Table'[idclient], 'Table'[idticket], 'Table'[Date_time]
) ) )
),
[num instances]
)

yes, that's right. I need to find repeated occurrences for the same idclient and ticket excluding the duplicate values)

Thank you  johnt75 , is it possible to show a table with the idclient, idticket and date_time for these occurrences?

I think you could add a calculated column as below, then filter out any entries which are 1

Is first entry = 
var currentTicket = 'Table'[idticket]
var currentClient = 'Table'[idclient]
var currentDateTime = 'Table'[Date_time]
var numPrevEntries = CALCULATE( COUNTROWS('Table'), REMOVEFILTERS(), 'Table'[idclient] = currentClient && 'Table'[idticket] = currentTicket 
&& 'Table'[Date_time] < currentDateTime )
return IF ( numPrevEntries = 0, 1, 0)

Hi johnt75 actually as a column does not work because there is no row context I guess but if I write this way almost work. The only problem is that the duplicates are also 1 in this formula:

I meant for you to use the calculated column so that you could display the data in a table, not to replace the num occurences measure.

By adding the new column as a filter the table will also obey any other filters or slicers you apply. e.g. using a slicer to select August produces

I know johnt75 , I didn't replace it. I used it as a filter. What I was trying to do is display the total for each client and the total for all in a table. That's why I said that if you tried to do with the calculated column the duplicates appear because they also have 1 in the condition.

You may be able to get rid of the duplicates if you replace the ALL('Table') with

CALCULATETABLE( SUMMARIZE( 'Table', 'Table'[idclient], 'Table'[idticket], 'Table'[Date_time]),
    REMOVEFILTERS( 'Table' )
)