You can use the for_each meta-argument to create multiple volumes.
Example
To create 3 different volumes use the following configuration:
resource "powerflex_volume" "volume_example" {
for_each = toset( ["Volume1", "Volume2", "Volume3"] )
name = each.key
protection_domain_name = "domain1"
storage_pool_name = "pool1"
size = 8
capacity_unit = "GB"
}