Can A level 11 Fighter multiclass into a ranger and level up to level 6 to get the most extra attacks before level 20? I’m pretty sure when multiclassing doesn’t stack the extra attacks, but I just want to make sure. Then the character would be in total level 17, (6 Ranger levels and 11 Fighter levels) with 4 extra attacks, as well as any path you chose perks/upgrades, right?
No, Extra Attack does not combine.
The multiclassing section in the PHB covers this quite explicitly:
If you gain the Extra Attack class feature from more than one class, the features don't add together. You can't make more than two attacks with this feature unless it says you do (as the fighter's version of Extra Attack does). Similarly, the warlock's eldritch invocation Thirsting Blade doesn't give you additional attacks if you also have Extra Attack.