Can my landlord stop my partner from staying over?Beta*

Generally not. It's your home and you can have visitors, including a partner who stays some nights. A landlord can reasonably object if someone has effectively moved in. That matters for council tax, and in an HMO for the licence limits on how many people can live there. They can also object if a visitor is causing damage or nuisance. Short of that, telling you your boyfriend or girlfriend comes over too often is not something the landlord can enforce. Keep things factual in writing. If someone is moving in for good, ask the landlord's permission to add them.

Sources: Shelter: your rights as a private tenant