Most individual house numbers or house number plaques come with matching screws only. We recommend purchasing standard plastic anchor inserts.
The house number screws will then screw into the plastic anchors.
Before drilling anything, measure twice and use a level (we cannot stress this enough). Use a masonry drill bit for drilling your holes. Use a smaller sized drill bit first and then widen with a drill bit sized appropriate for the plastic anchors.
You can squirt some silicone adhesive or exterior grade silicone caulk into the holes to hold the anchors in more securely if you like
Now...here is the challenge....
The stucco surface is uneven.
So what you can do is use some very small washers (around the screws and behind the numbers or plaque) to raise the numbers or plaque off from the stucco surface. You can play around with this using more washers on some screws than others in order to get an even look. Try and use brass washers if possible.
The result will be sort of a "floating" look as if it were mounted with stand offs.
But this is a far better look than a haphazard uneven appearance.
We also offer Craftsman style house number backer boards for mounting individual house numbers.