My main project has over 240 inputs, including audio, video, images, playlists with multiple items, numerous NDI inputs (not all of which necessarily receive a stream, but the inputs are constantly pre-loaded), and many GT Titles. Four NDI outputs are always active. The project is running at 1920x1080 25fps and I haven't experienced any noticeable slowdowns.
My config : Alienware m18 R2 (Intel Core i9-14900HX, Nvidia GeForce RTX 4090 (16Go GDDR6), 32Go RAM DDR5-5600MHz)

I've tested well over 350 Full HD videos, together with images, virtual sets and other production assets, on an Intel i7-9700 (3.0 GHz) with 16 GB RAM, and vMix has handled it surprisingly well.

In my experience, the practical limit depends much more on the type of inputs (NDI, videos, camera feeds, browser inputs, etc.), codecs, resolution and GPU performance than on the number of inputs alone.

If the layout contains a lot of PiP videos, you may notice some slowdown, but that's more of a PC performance limitation than a vMix limitation.

For my workflow, I haven't found the number of inputs to be the limiting factor.