Jace's Phantasm — ruling 1

The last ability of Jace's Phantasm constantly monitors each opponent's graveyard to see if the bonus applies. If the stated condition becomes no longer true, the bonus immediately stops applying.

Jace's Phantasm — ruling 2

If Jace's Phantasm is dealt lethal damage at the same time as an opponent's creature, and that creature will become that opponent's tenth card in their graveyard, Jace's Phantasm will be destroyed before it gets +4/+4. This is because all creatures that have been dealt lethal damage are destroyed at the same time.

Jace's Phantasm — ruling 3

In a multiplayer game, a single opponent must have ten or more cards in their graveyard for the bonus to apply, although this opponent may change over the course of the game. You don't have to pick a single opponent; the ability will monitor each opponent's graveyard.

Jace's Phantasm : règles, fonctionnement et interactions (MTG)

Jace's Phantasm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying This creature gets +4/+4 as long as an opponent has ten or more cards in their graveyard.

How it works : Jace's Phantasm

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
The last ability of Jace's Phantasm constantly monitors each opponent's graveyard to see if the bonus applies. If the stated condition becomes no longer true, the bonus immediately stops applying.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
If Jace's Phantasm is dealt lethal damage at the same time as an opponent's creature, and that creature will become that opponent's tenth card in their graveyard, Jace's Phantasm will be destroyed before it gets +4/+4. This is because all creatures that have been dealt lethal damage are destroyed at the same time.
No other card is named in this ruling.
#3November 17, 2017· Wizards of the Coast
In a multiplayer game, a single opponent must have ten or more cards in their graveyard for the bonus to apply, although this opponent may change over the course of the game. You don't have to pick a single opponent; the ability will monitor each opponent's graveyard.
No other card is named in this ruling.