How to count the number times a list repeats and adding to column in pandas dataframe

I have a dataframe with two columns 'Plate Number' and 'Well ID'.

The well_id column was populated using a repeated list = [A2, A3, A4, B2, B3, B4] now I need to find a way to to change the plate number everytime this list repeats by adding 1 so it produces the following dataframe.

I am kind of at a loss on how to do it, any advice on how to move forward with this in python?

hitpick_df['count'] = 0

# Iterate over the rows and increment the count
i = 1
for i in range(1, len(hitpick_df)):
  if hitpick_df['Destination'][i] == hitpick_df['Destination'][i - 1]:
    hitpick_df.loc[i, 'count'] = hitpick_df.loc[i - 1, 'count'] + 1
  else:
    hitpick_df.loc[i, 'count'] = 1

# Print the DataFrame
print(hitpick_df)

What resulted was a column full of 1's

Try cumcount

df['plate_number'] = df.groupby('well_id').cumcount() + 1

output:

   well_id  plate_number
0       A2      1
1       A3      1
2       A4      1
3       B2      1
4       B3      1
5       B4      1
6       A2      2
7       A3      2
8       A4      2
9       B2      2
10      B3      2
11      B4      2
12      A2      3
13      A3      3