according to article i, section 2, of the constitution, how were slaves to be counted when determining the number of congressional districts per state?

They were to be counted as three‑fifths of a person for purposes of determining representation in the House (and thus how many congressional districts/Seats each state received).

In the language of Article I, Section 2, the population base for apportioning representatives was defined as the “whole Number of free Persons” plus “three fifths of all other Persons,” a euphemism for enslaved people. This arrangement became known as the Three‑Fifths Compromise and meant that only 60% of each state’s enslaved population was added to its population total when assigning House seats.