The free version limits the number of GPUs to a single card. It fully utilises that GPU so the only limitation is adding extra GPUs.
Here’s what they say on the support page where you download the software –
” The free DaVinci Resolve 16.1 includes all of the same high quality processing as DaVinci Resolve 16.1 Studio and can handle unlimited resolution media files. However it does limit project mastering and output to Ultra HD resolutions or lower. DaVinci Resolve 16.1 only supports a single processing GPU on Windows and Linux and 2 GPUs on the latest Mac Pro.
If you need features such as support for multiple GPUs, 4K output, motion blur effects, temporal and spatial noise reduction, de-interlacing, HDR tools, camera tracker, multiple Resolve FX, 3D stereoscopic tools, remote rendering, an external database server and collaboration tools that let multiple users work on the same project at the same time, please upgrade to DaVinci Resolve 16.1 Studio. “