How do I assign index like this after groupby pandas?

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Python

data-science

I have this Dataset:

Accession Number	Value
0	0001754960-22-000233	258
1	0001754960-22-000233	254
2	0001754960-22-000233	254
3	0001754960-22-000233	259
4	0001754960-22-000233	3753
...	...	...
2568329	0000950123-22-007614	13660
2568330	0000950123-22-007614	25464
2568331	0000950123-22-007614	1368
2568332	0000950123-22-007614	31956
2568333	0000950123-22-007614	10224

I want Output Where, every Accession Number has S.no which will assign the values, for eg: Accesssion Number -0001754960-22-000233 will have S.no as 1 for all rows, Next Accession will have 2 for all it's rows and so on.


Somewhat like this:

Accession Number	Value	S.No
0	0001754960-22-000233	1	
1	0001754960-22-000233	1	
2	0001754960-22-000233	1
3	0001754960-22-000233	1
4	0001754960-22-000233	1	
5	0001754960-22-000233	1

What I have tried:

df5['Index']=df5.groupby('Accession Number')['Accession Number'].transform('count')