How to get the frequency of column depending on certain values of another column

Question

I have a DataFrame with multiple columns. The DataFrame is unique with respect to col_a. I want to find out the frequency of col_c against each unique value of col_b. Note that some of the values of col_a are repeated in col_b. All the columns are of the type object.

Example:

col_a col_b col_c
1234  1234  1567
1235  1234  1567
1236  1234  1987
1237  1234  1786
1321  1321  986
1322  1322  356
1323  1322  356
1354  1354  356
1364  1364  986

Output expected:

    col_a col_b col_c freq_c
    1234  1234  1567   2
    1235  1234  1567   2
    1236  1234  1987   1
    1237  1234  1786   1
    1321  1321  986    1
    1322  1322  356    2
    1323  1322  356    2
    1354  1354  356    1
    1364  1364  986    1

I have already found the frequency of col_c with against each unique value of col_a by using the following command:

df['freq'] = df['col_c'].map(df['col_c'].value_counts())

But it doesn't work for col_b as the df is not of the same length as col_b.

Question: How to find the frequency of col_c against each unique value of col_b?