The spacer is governed by the stud length unless you get the hub centric wheel spacers with studs
and that would be governed by the size of the tires and wheels you're going to use.
If it's the stock assembly, just measure the distance you want to move the tire out, and that would be the spacer thickness.
You can measure the distance between wheels and fenders to determine what thickness of wheel spacers to install.
I had BONOSS wheel spacers 20mm on my car for almost 21,000 miles, with no issues whatsoever.
Maybe you can try what I installed.
They can customize the thickness and data that I want, so that I can install new tires with different data, and the customization time is very fast, just 1~3 days.
They have aluminum alloy 7075-T6 and 6061-T6 options. 7075-T6 will be more expensive but better in quality. In fact, 6065-T6 is good enough. Its tensile strength, yield strength and elongation can support our normal use. And its lugs are grade T10, studs are forged grade 12.9.