How much memory and vCPU should be allocated per one offload scan server
4 CPU + 4 GB memory
does offload scan server must be on the same Xenserver
No. You don't want them seperated by large distances but they can be on different servers
we have to purchase MOVE licenses per desktops, or are there also licenses for offload scan server?
You are fully licensed for the scan servers now. No matter how many you use.
What is recommended number of offload scan servers in case of 100, 1000, 5000 desktops
You will have to test how many works best for you and what scheme you use to load balance. Start with a pair of scanner per 250 VDI as a baseline.