Flow formula: business days between dates

/* Formula (Number) — business days from {!startDate} to {!endDate} */
(5 * FLOOR(({!endDate} - DATE(1900, 1, 8)) / 7)
  + MIN(5, MOD({!endDate} - DATE(1900, 1, 8), 7) + 1))
-
(5 * FLOOR(({!startDate} - DATE(1900, 1, 8)) / 7)
  + MIN(5, MOD({!startDate} - DATE(1900, 1, 8), 7) + 1))

No Apex, no loops — one formula resource that counts weekdays between two dates.

Gotchas

DATE(1900, 1, 8) is a Monday — that anchor is what makes the modulo arithmetic work. Holidays are not weekends: if you need them excluded, that is a custom metadata lookup, not a formula.