How will the email look with multiple items in it?

Most of the time when previewing an email, you'll see what the email looks like with one item in the email. What you can do to see how it looks with different variations is to go into the Pending tab and preview some of the emails to various customers - that should give you a chance to see what it looks like in different scenarios.

The way that the item grid works is that it adapts to the number of items in the email. If there is only one, it's displayed at a bigger size.

If there are two, the size is slightly smaller

And with more than four items, they're even smaller.

Also there are a few template variables that are based upon the number of items in the cart. The {{this_item}} for example shows either "this item" or "these items" depending on whether there is one item or multiple items.

In the quick preview, it will just show "these items", but if you go into the Pending tab and preview an actual pending recipient's email, you can see what it will look like.