Usually two, and the number is the easy part. What actually happens when you add a second animal: fresh approval, per-pet charges, and a new noise profile.
No pet fee, no pet deposit, no pet rent, and restrictions generally waived. Also: HUD's enforcement position has reportedly shifted, so check it today.
Lease alteration clauses are broader than renters think, and an unapproved install can become the landlord's property with restoration billed to you.
Ask about the attachment, not the device. The four things a landlord is actually worried about, answered before they raise them, plus what to write.
Adhesive hooks hold about 7.5 lb. A two-gallon pet fountain is heavier than that before you add the housing. The arithmetic no mounting guide does.