The Warden Ranger archetype says this:
Master of Terrain (Ex): At 1st level, a warden selects his first favored terrain. He gains another one at 3rd level, and then every five levels thereafter (for a maximum of five favored terrains at 18th level). This ability replaces the ranger’s first favored enemy ability.
That's all well and good for a single classed Warden. But if they were to take only 10 levels and then became a Horizon Walker, they could get 3 from Warden, and 7 more from Horizon Walker.
Does that mean they are limited to 5 NO MATTER WHAT or 5 from Warden, but can get more from other viable combinations?
It means the latter. The cap is only mechanically meaningful if the campaign goes past 20th level and you stick with Warden Ranger the whole way. Otherwise it's just there to confirm your understanding of the math.