The groupby function can be used to group (and count) the number of records in a data frame that meet your criteria.

Continuing with our Titanic dataset, we can use groupby to count the number of people by age.

We can use the following script:

# read in the titanic data set
import pandas as pd
df = pd.read_excel('http://biostat.mc.vanderbilt.edu/wiki/pub/Main/DataSets/titanic3.xls')
# extract just the age column to its own dataset, 
# group by the age, and
# add a count for each age
ages = df[['age']].groupby('age')['age'].count()
print (ages)  

The resultant display under Jupyter is as follows. I had not realized there were so many babies on board.