1.4 or 6 waves depending on how you view the battle
The DMG says that a hard encounter for 4 8th level characters should be worth 5600 XP. A single wave of 20 zombies is worth 1000 XP, but gets a x4 multiplier for a total of 4000 XP (DMG 82). Thus, by the book, 1.4 waves is enough for a hard encounter.
It's also worth noting that DMG 83 suggests that you treat each wave as an individual encounter:
For such encounters, treat each discrete part or wave as a separate encounter for the purpose of determining its difficulty.
If you do this, the total adventuring day XP budget for your party will be 24,000 XP, which means that your party should face about 6 waves for the entire day.
You'll need way more than that
In my experience, the DMG's encounter guidelines work best when the PCs are fighting a group of similar size--4 PCs vs. 4 monsters, for example. The further you get from a 1:1 ratio, the more likely that specific traits will make a huge difference. For example, 1.4 waves will be easy for this encounter--a single fireball will probably kill the first wave, and a turn undead would mop up the second. Your wizard can cast up to 5 fireballs (and sculpt spells), so that's probably the minimum number of waves you need before the encounter becomes anything but trivial.
Therefore, the easiest way to make a challenging encounter is to just keep throwing waves at them until the PCs begin to run out of resources. Even if you do set a hard limit on the number of waves, your players are never going to know if you planned for 2, 5, or 50 waves anyway. However, you will know if your evoker runs out of fireballs or the party starts to get overwhelmed, so you can just choose not to send in another wave after that. That's probably the best way to simulate an exhausting battle of attrition, especially if you're not going to use things like tactics or terrain.