If I have a shadow clone that attack 2 times in a turn will it have assault 8 or assault 4 ?

Assault 8 — if both triggers resolve.

Shadow Clone's token ability is a triggered "when I attack" that grants it Assault 4 this turn. Since that bonus lasts this turn, and Assault values from multiple sources sum (Rule 807.2), two successful triggers (banish a unit from your trash both times) give the clone Assault 4 + Assault 4 = Assault 8 while it's an attacker on the second attack. It would only be Assault 4 during the first combat.

This differs from printed Assault on cards like Lucian, which is a passive ability that simply toggles on/off each combat and doesn't accumulate. If you can't pay the banish cost on the second attack, you'd keep only the first Assault 4.