Hi Jenny,
Yes, conditions are limited to 5 comparisons.
But, why not just use the out-of-box support. Click on the fields in taskpane and select the checkbox "cannot be blank". Clicking submit will show an error and tell users the form is incomplete.
To make the fields stand out, one cool thing I do is just add a formatting rule if the field is blank to highlight the background in yellow.
That way, users will know what's left.
Easy and out-of-box wins in the end!
:)