Determine the first occurrence of a particular customer visiting the store in a particular month - sql

Question

I need to determine the counts breakdown to per month (and year) of customers [alias'ed as Patient_ID] which made their first visit to a store. The date times of store visits are stored in the [MDT Review Date] column of the table.
Customers can come to the store multiple times throughout the year and increase the total count-> but what I require is ONLY the first time a customer visited.
E.g. Tom Bombadil visited the store once in January 2019, so count increased to 1, then again 4 times in March, so count should be 1 for the month of March and 0 for febraury and 1 for January, then again 4 times in October, then again 2 times in December.
I require that Tom Bombadil should be counted one and only once for a particular month, his first occurrence which was per month
The output should be like :
rn1 YEAR Month_Number Month Total_Count
1 2010 6 June 2
1 2010 7 July 1
1 2010 8 August 5
1 2010 10 October 5
1 2010 11 November 3
1 2011 1 January 4
1 2011 2 February 6
1 2011 4 April 7
1 2011 5 May 4
1 2011 6 June 10
1 2011 7 July 10
1 2011 8 August 14
1 2011 9 September 4
1 2011 10 October 8
1 2011 11 November 11
1 2011 12 December 11
1 2012 1 January 8
1 2012 2 February 21​
Please refer to my query. What I have attempts to use the windowing function COUNT to count the store visits per month. Then the ROW_NUMBER function attempts to assign a unique number to each visit. What am I doing wrong?
select
*
from
(select distinct
row_number() over (partition by p.Patient_ID, p.PAT_Forename1, p.PAT_Surname
order by PAT_Forename1, p.Patient_ID, PAT_Surname) AS rn1,
datepart(year, [DATE_COLUMN]) as YEAR,
datepart(month, [DATE_COLUMN]) as Month_Number,
datename(month,[DATE_COLUMN]) as Month,
count(p.Patient_ID) over (partition by datepart(year,[DATE_COLUMN]),
datename(month, [DATE_COLUMN])) as Total_Count
from
Tablename m
inner join
TableName p on m.PK_ID = p.PK_ID
) as temp
where
rn1 = 1​