A non-rare guard tower will spawn 1 guard per 10ql up to 5 guards. Rare guard towers will spawn 1 additional guard per rarity as long as the quality requirement is met; as per the table below.

NOTE: Tower guards will not attack hell horses, but settlement guards will unless you adjust settlement settings to keep them from attacking aggressive targets, but doing so will negate the need for settlement guards as they will not attack anything aggressive.