The honest answer: at least one, usefully two or three
A single cat needs one proper scratching post as an absolute floor, but scratching serves several jobs at once, and one post in one corner rarely covers them all. Cats scratch to maintain their claws, to stretch their shoulders and back, to leave scent from glands between their toes, and to mark a place as theirs. Because marking is about location, a cat whose only post sits in a spare room will still feel the need to sign the sofa. Two or three options, placed where the scratching impulse actually happens, serve a cat far better than a single expensive post in the wrong place.
Put posts where the urge strikes
The two highest-value locations are beside where your cat sleeps and beside the furniture she already targets. Cats scratch almost reflexively on waking, as part of a full-body stretch, so a post within a few steps of her favourite bed catches that daily session. The second post belongs next to the sofa arm, chair leg or door frame she would otherwise use, because you are offering a legal alternative at the exact scene of the crime. Redirecting works when the replacement is closer and better than the original, not when it is in another room entirely.
What makes a post worth using
Stability is everything. A cat leans her full weight into a scratch, and a post that wobbles or tips will be abandoned, sometimes after a single scare, in favour of solid furniture. For an upright post, choose one tall enough for a complete vertical stretch with claws extended above her head, on a heavy, wide base. Coverings matter too: tightly woven sisal gives satisfying resistance for most cats, while some prefer sisal fabric, corrugated cardboard laid flat, or carpet. Offering two different textures across your posts lets her tell you her preference, and horizontal scratchers suit cats who naturally scratch rugs and carpets rather than chair legs.
Retraining a cat who already uses the furniture
Do not punish scratching; it is a need, not naughtiness, and punishment only teaches her to do it when you are out. Instead, make the furniture temporarily less appealing with a throw or a fitted cover, place the post immediately beside it, and reward her with praise, play or a treat the moment she uses the post. Rubbing a little catnip on a new post, or playing with a wand toy around its base so her claws catch it mid-game, helps many cats make the connection. Once the habit has moved, you can edge the post gradually toward where you would prefer it to live.
Replacing and maintaining posts
A post shredded down to loose strands has done its job and needs replacing rather than celebrating, because worn-out sisal no longer gives her claws useful resistance. Check wooden posts and wall-mounted scratchers for splinters or exposed fixings as they age, and replace horizontal cardboard pads once both sides are torn through. If scratching suddenly increases or shifts to new places alongside other changes in behaviour, mention it at her next vet visit, since discomfort and stress can both show up as restless marking.
Redirecting her consistently at home
Retraining fails when one person redirects her to the post and another lets her shred the armchair in peace. Opening her profile to a grandparent or adult child you have picked is possible under KinsLe Family Sharing.