Azure determines which Spot Virtual Machines to evict based on two primary factors: capacity requirements and the maximum price you've set for your Spot VMs.
When Azure needs to reclaim compute capacity for other workloads, it may evict Spot VMs.
If you've set a maximum price for your Spot VMs, they can also be evicted if the current Spot price exceeds your specified maximum.
In your scenario, with 100 Spot VMs of the same type, all configured with a max-price set to -1 (indicating no price limit), the likelihood of all your VMs being evicted simultaneously depends on Azure capacity demands.
If Azure experiences a significant surge in demand for the specific VM size in your region, it's possible that multiple or all of your Spot VMs could be evicted at the same time.
However, Azure does not publicly disclose the exact algorithms or selection criteria it uses for evicting specific Spot VMs.
https://learn.microsoft.com/en-us/azure/virtual-machines/spot-vms
https://learn.microsoft.com/en-us/azure/architecture/guide/spot/spot-eviction