I think I know why Asta has five horns. It is the same reason Zenon became a devil himself.

Asta does have his own source of anti magic now. Zenon said he would give his soul in exchange for a devil heart. A trade.

Asta did the same thing. He traded his human arm for power.

Now many use the fact that Asta lost his arm when Liebe appeared as evidence against my point. However I don't think it's like that. In the last chapters you can see Asta and Liebe both have changed arms.

If Asta's arm was simply given back after the devil binding ritual there is no reason that Liebe should have another.

So what I think happened is that Asta now has his own source of power.

Now to further prove my point , we must remember the two times Asta powered up willingly, in the lava training field and the first time he amped the demon destroyer.

Both times he could not control the anti magic at all. Ended up smacking Lumiere and hunting the lioness.

Heck even, when Zagred showed up the anti magic was resonating with him.

That is what I believe happened to Asta's arm during match with Liebe. Asta's anti magic arm resonated with another source of anti magic and was disrupted. Plus the glow or anti magic never disappeared from Asta's arm.

That is why Asta has a fifth horn. He had become part devil.

Now, what does this mean for the future? The reason why I bring up this is because Tabata follows up. Charlotte's curse, Tabata followed that up with the Vanica fight and we all know what happened there.

Asta's new power not only will support devil union but will most likely lead to an easier time handling what I believe will be his new sword. A combination of all previous swords, a holy sword to strike down evil.

Power directly from a devil, even through Asta may not pair well with it. However if that power is diluted with Asta's own magic I believe it will work.

Now this last part is pure speculation, however I do firmly believe Asta has his own source of Anti magic now.