Four, all of them can be added to your harem. Two become widows after it starts (not the MC's fault for either) and one husband knows you're cucking him. The last husband never appears in-game and we don't know if he knows. Opinions differ on whether the ogress counts, and she's the only one where you actively get rid of her partner. The goblin wives don't count IMO, their culture is...different.
All of them have reasons for cheating on their partners (usually selfish/envious/morally questionable, but even so) and knowingly take up with the MC. If you're looking to actively steal a loyal woman from someone else PQ will probably disappoint.
None of these have to be done for the Main storyline. But if you don't you miss out on a lot of quests, enthusiastic sex partners, and non-sex benefits.