The wheel diameter is the least of the 3 variables you need to consider. The other 2 are offset and wheel width. These 2 #s are found by the valve stem. With offset & width info, you can use a calculator like the link to determine how your 'new' wheels will sit compared to your existing ones (you need the same info on your current wheels).
https://www.1010tires.com/Tools/Wheel-Offset-Calculator
In most cases, a 911 front wheel will work on a 986 but the 911 rear will need a wildly big spacer to make it clear on the inside. Then it will be pushed too far out and create clearance issues on the outside.
The biggest factory diameter wheel on a 986 was 18". Most guys ask about using 18" 911 wheels. Think you're the first to ask about 20s. You may run into issues with suspension components with 20s. I wouldn't recommend going that big or using a 911 wheel. Research before you buy an expensive set of lawn ornaments.
Do a search on
offset to read up on it. There are LOTS of threads with your same question.