One suitable tree may be enough for one cat, but there is no mandatory number. What matters is the range and location of opportunities available around the home. A cat that spends time on different floors may appreciate a resting or scratching option in more than one area, without needing a complete tower in each room. Observe where it already chooses to sleep, stretch and watch activity. A single tree placed in an unused corner may offer less value than a simpler setup in a preferred location.
Before buying another large item, consider whether moving the existing one or adding a separate scratcher would solve the problem. Also leave space for ordinary movement and cleaning. The goal is accessible, well-used resources, not filling the house with equipment simply to meet an imagined quota. Record where the cat chooses to spend time over a few days before adding another tower; repeated use of a location is more useful than an empty corner.
For product comparisons alongside this advice, see Ultimate PawHouse and Taj-MeowHall. The details below explain their sizes and uses.
Further reading: cats.org.uk; rspca.org.uk.
Ultimate PawHouse
Taj-MeowHall
Cat-Cave