At first the tails come out unconsciously as Naruto gets angry and loses it. The ease at which it comes out is directly related to just how emotional and urgent he is in need for power (aka shounen heart).

Before the time jump, he had two tails when trying to stop Sasuke leaving at the waterfall. I'd say that's his "spirit" at its max. After the time jump, he got the opportunity to see Orochimaru, which is pretty much the key to Sasuke. Again, at the same "max" we could say, he released up to 4 tails.

His emotions are the key to releasing the tails, but it's the weakening seal that controls exactly how many he can get to. As Jiraiya said, ultimately there'll be nine.

From what we've been shown, 4 tails Naruto completely loses it. He never held back at all. I can't tell if Naruto's emotions then are the trigger anymore, since the fox is now in control. I can only guess if the seal weakens and allows more tails, it (fox) would definitely make it emerge in its attempt to devour Naruto.