The WotC article Unearthed Arcana: Modifying Classes creates the "Favored Soul" sorcerous origin, which grants the the sorcerer the Extra Attack feature at level 6.
Say I made Favored Soul sorcerer go up to level 6, and then multiclassed into Ranger to get its Extra Attack at level 5. Would the two Extra Attack features stack so that I could get 3 or 4 attacks in a turn?
The rules for stacking Extra Attack as they relate to non-fighters and fighters are in the Multiclassing section of the Player's Handbook, pg. 164:
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.
The only class that can expand the ability of Extra Attack to make more than 2 attacks per Attack action is the Fighter. The Fighter's Extra Attack feature says (PHB, pg. 72):
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.
The number of attacks increases to three when you reach 11th level in this class and to four when you reach 20th level in this class.
PHB, page 164:
Extra Attack
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.
Regardless of gaining it at level 5 or 6, any class feature that gives you multiple attacks on your attack action does not stack with another class feature with the same or similar effect. Fighter is currently the only route to go to have 3 or 4 attacks solely on the attack action.
