How to sum up how many elements of the same value for a specific field in a pandas dataframe python?

Question

I have a pandas dataframe called hospitals and there are fields e.g number of employees and city that it is in.

I want to sum up how many hospitals are in each city and store that as a new column or field in the dataframe.

Tried this which doesn't work:

    print(df.groupby('city').agg({
    'city': pd.Series.nunique
}))

    hospital_id    name                    city
0             1    Cassel Hospital         London
1             2    Oaklands Hospital       Manchester
2             3    Bromley Hospital        London

I want it to be like this:

        hospital_id    name                    city    Count
0             1    Cassel Hospital         London       2
1             2    Oaklands Hospital       Manchester   1
2             3    Bromley Hospital        London       2