Can I do GPU passthrough with only one GPU?

i can confirm that on Hyper-V, Linux and on ESXi you can pass-through any/all GPUs that you have, this includes your question (assuming only one GPU exist). the host simply has no display anymore (that's called headless in the server world) but why would it care?

RDP/SSH/Web/remotePowershell access still works.

i started these tests two years ago with hyper-v (server 2016) and an old radeon R390 and a radeon RX480. because of driver issues i later switched to linux (centos 7/8) with KVM. while driver issues were solved, the linux solution was wonky because of cpu scheduling issues which resulted in heavy usb audio glitches. last week i tried out hyper-vserver 2019, but abandoned the project because it seems server2019 no longer allows consumer grade GPUs to pass-through. instead of wasting too much time i tried out ESX, this is my preferred solution so far.

i currently run esxi 7 with a RX480 passed to one VM, and an old nvidia quadro400 to the second vm. host is headless = no GPU. during this switch i made tests with one GPU installed only, and passthrough works fine in this case as well.

i hope this answers your question.