https://mtgdecks.net/guides/mastering-izzet-phoenix-by-lee-shi-tian-mtg-306

General Mulligan Guide
Rule One
I generally avoid keeping one-landers on seven cards. This deck is exceptionally good at using all available mana thanks to its suite of cantrips, so while there’s usually a 60-80% chance of hitting the second land drop, I’m not willing to risk the roughly 20% chance of giving my opponent a free win due to a missed land drop.

Rule Two
Beyond ensuring a minimum of two lands, the next key to a keepable hand is at least two one-cost cantrips, Picklock Prankster, or Artist’s Talent. This deck shines when it flows smoothly, and a hand with only interaction spells won’t get you very far. Early threats, besides Artist’s Talent, aren’t crucial; you will draw into threats naturally as the game progresses.

While some games will inevitably see all your Arclight Phoenixes buried in the bottom 20 cards, there’s no way to anticipate that in a mulligan decision. A hand clogged with too many Phoenixes, however, is a definite way to lose, so aim for balanced draws that enable the deck’s consistent pacing.

vs Demon Variant (you can tap out)
+2 Abrade, +1 Proff's Eidetic, +1 Spell Pierce, +1 Annul, +1 Brazen Borrower
-3 Fiery Impulse, -1 Torch the Tower, -1 Lightning Ax, -1 Galvanic Iterat

vs Fable Variant with Combo (you cannot tap out)
+2 Abrade, +1 Spell Pierce, +1 Annul, +1 Brazen Borrower
-1 Temporal Trespass, -1 Galvanic Itera, -1 Torch the Tower, -1 Lightning Axe, -1 Into the Flood Maw