More SQL Query questions

Dear expert

Please help me with a query command you can find below, Now I got problem with variables @P4, @P5, @P6, problem is that I want to show result that minus the value of @P1, @P2, @P3, this  R.DatetimeSentToReceiver < DATEADD(MONTH, 2, @ReminderMonth) adds 2 months value, what i want is @P4 = @P4 - @P1 now I tried add this in query, and it return with error ofc. How to do this in the correct way? There are many way doing this, I need a great solution for it, what Im after is this, data in current months output, data in last month output.  @ReminderMonth will be every months day 1 so that will change too.
 
Any help would be great.

      DECLARE
            @ReminderMonth date = '2018--01-01';

DECLARE @P1 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 1, @ReminderMonth)
        AND R.ReminderNumber = 1
            ),
        @P2 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 1, @ReminderMonth)
        AND R.ReminderNumber = 2
            ),
        @P3 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 1, @ReminderMonth)
        AND R.ReminderNumber = 3
            ),
        @P4 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 2, @ReminderMonth)
        AND R.ReminderNumber = 1
        ) ,
        @P5 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 2, @ReminderMonth)
        AND R.ReminderNumber = 2
            ) ,
        @P6 int =  
        (
            SELECT      
        COUNT(*)
        FROM Invoice.Reminder R
        WHERE
        R.DatetimeSentToReceiver >= @ReminderMonth
        AND R.DatetimeSentToReceiver < DATEADD(MONTH, 2, @ReminderMonth)
        AND R.ReminderNumber = 3
       ) ;


        SELECT CONVERT(varchar(7),@ReminderMonth,120) AS 'Current month', @P1 AS 'Reminder 1', @P2 AS 'Reminder 2', @P3 AS 'Reminder 3', SUM(@P1+@P2+@P3) AS 'Curry month reminder Total',