What is the minimum number of visit required to deliver a package in the apartment community?

Time Availability = [[1,6],[2,3],[2,5],[4,6],[6,7],[6,8]]
Total Number of apartments = 6
Minimum number of visits = 2

Apartment 1,2,3 => 2 (1st visit)
Apartment 4,5,6 => 6 (2nd visit)