You should go with 16 GB. macOS allocates up to 1.5GB for GPU, so 8GB RAM could be as low as 6.5GB from the get go. Windows 10 needs minimum of 4 GB to run well, leaving only 2.5 GB for macOS and its applications.
Because MacBook Air has such a speedy SSD, if you only need to run VM on occasions, you might be able to get by with 8GB. But don't expect smooth performance.