I have deployed fleet on Kubernetes, should I use Redis cluster or a single instance?

Fleet is compatible with either. Really depends on scale (how many hosts are registered, and how frequently are large live queries run), and if you feel like you need Redis deployed in a highly available manner. Generally I’d say it’s perfectly fine to use a single instance unless you need to run cluster mode for the stated or other reasons.

i mean I'm having something like 10000 hosts online at max

I think a single node is fine with adequate resources