Count Orders Per Customer

Counting rows per group is the simplest aggregation and the one with the most ways to get a slightly wrong answer, because Pandas offers four spellings of it that do not all mean the same thing.

Write order_counts(df).

df has columns order_id and customer_id, one row per order. Return a DataFrame with columns customer_id and order_count, one row per customer, holding how many orders that customer placed.