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While subnetting Class B address how to find out subnets number?

In this query why do we subtract 2 from 2^three whilst calculating some subnets? Whilst in CCNA books 2 isn't always subtracted?

172.16.0.0 /19

Subnets 2^3 -2 = 6

Hosts 2^13 -2 = 8190

Block Size 256-224 = 32