xe vm-param-list uuid=<VM_UUID> | grep -i vcpu

where you replace <VM_UUID> with the actual UUID of the VM. The output should look similar to this:

# xe vm-param-list uuid=dc7beebf-46e0-ab98-1743-e0324e24xxxx | grep -i vcpu
                          VCPUs-params (MRW):
                             VCPUs-max ( RW): 2
                      VCPUs-at-startup ( RW): 2
                    allowed-operations (SRO): sysprep; metadata_export; changing_VCPUs_live; changing_dynamic_range; migrate_send; pool_migrate; suspend; hard_reboot; hard_shutdown; clean_reboot; clean_shutdown; pause; checkpoint; snapshot
                      last-boot-record ( RO): '('struct' ('uuid' '2c74c10c-325c-8c54-63d9-8c83e54abf63') ('allowed_operations' ('array')) ('current_operations' ('struct' ('OpaqueRef:dd2f6f24-427c-de26-2d1b-744f25f2f001' 'start'))) ('power_state' 'Halted') ('name_label' 'Debian 12 Cloud-init (Hub)') ('name_description' 'Debian 12 Cloud-Init Ready Hub Template') ('user_version' '1') ('is_a_template' ('boolean' '0')) ('suspend_VDI' 'OpaqueRef:NULL') ('resident_on' 'OpaqueRef:NULL')
('affinity' 'OpaqueRef:NULL') ('memory_overhead' '20971520') ('memory_target' '2147483648') ('memory_static_max' '2147483648') ('memory_dynamic_max' '2147483648') ('memory_dynamic_min' '2147483648') ('memory_static_min' '2147483648') ('VCPUs_params' ('struct')) ('VCPUs_max' '2') ('VCPUs_at_startup' '2') ('actions_after_shutdown' 'destroy') ('actions_after_reboot' 'restart') ('actions_after_crash' 'restart') ('consoles' ('array')) ('VIFs' ('array' 'OpaqueRef:d2cbfa49-86ff-3497-2843-ca1932509b95')) ('VBDs' ('array' 'OpaqueRef:3f7aa90c-9f45-f25d-9bce-5936cd010495' 'OpaqueRef:c8b90a71-a76b-bbdc-fd69-9184e51b7255')) ('crash_dumps' ('array')) ('VTPMs' ('array')) ('PV_bootloader' '') ('PV_kernel' '') ('PV_ramdisk' '') ('PV_args' '') ('PV_bootloader_args' '') ('PV_legacy_args' '') ('HVM_boot_policy' 'BIOS order') ('HVM_boot_params' ('struct' ('order' 'cd') ('firmware' 'bios'))) ('HVM_shadow_multiplier' ('double' '1')) ('platform' ('struct' ('pae' 'true') ('apic' 'true') ('timeoffset' '0') ('acpi' '1') ('viridian' 'false') ('nx' 'true'))) ('PCI_bus' '') ('other_config' ('struct' ('base_template_name' 'Debian Jessie 8.0') ('import_task' 'OpaqueRef:5403e9af-b418-bbf8-19f8-f2676b332685') ('mac_seed' '71148202-a0da-6490-164c-1c6311d09bf9') ('install-methods' 'cdrom,nfs,http,ftp') ('linux_template' 'true'))) ('domid' '-1') ('domarch' '') ('last_boot_CPU_flags' ('struct')) ('is_control_domain' ('boolean' '0')) ('metrics' 'OpaqueRef:d2f3433e-2263-ac9a-b45a-b8de124c9373') ('guest_metrics' 'OpaqueRef:NULL') ('last_booted_record' '') ('recommendations' '<restrictions><restriction field=\"memory-static-max\" max=\"1649267441664\"/><restriction field=\"vcpus-max\" max=\"32\"/><restriction field=\"has-vendor-device\" value=\"false\"/><restriction field=\"allow-gpu-passthrough\" value=\"1\"/><restriction field=\"allow-vgpu\" value=\"1\"/><restriction max=\"255\" property=\"number-of-vbds\"/><restriction max=\"7\" property=\"number-of-vifs\"/></restrictions>') ('xenstore_data' ('struct')) ('ha_always_run' ('boolean' '0')) ('ha_restart_priority' '') ('is_a_snapshot' ('boolean' '0')) ('snapshot_of' 'OpaqueRef:NULL') ('snapshots' ('array')) ('snapshot_time' ('dateTime.iso8601' '19700101T00:00:00Z')) ('transportable_snapshot_id' '') ('blobs' ('struct')) ('tags' ('array')) ('blocked_operations' ('struct')) ('snapshot_info' ('struct')) ('snapshot_metadata' '') ('parent' 'OpaqueRef:NULL') ('children' ('array')) ('bios_strings' ('struct')) ('protection_policy' 'OpaqueRef:NULL') ('is_snapshot_from_vmpp' ('boolean' '0')) ('appliance' 'OpaqueRef:NULL') ('start_delay' '0') ('shutdown_delay' '0') ('order' '0') ('VGPUs' ('array')) ('attached_PCIs' ('array')) ('suspend_SR' 'OpaqueRef:NULL') ('version' '0') ('generation_id' '') ('hardware_platform_version' '0') ('has_vendor_device' ('boolean' '0')) ('requires_reboot' ('boolean' '0')) ('reference_label' 'debian-8'))'
                       recommendations ( RO): <restrictions><restriction field="memory-static-max" max="1649267441664"/><restriction field="vcpus-max" max="32"/><restriction field="has-vendor-device" value="false"/><restriction field="allow-gpu-passthrough" value="1"/><restriction field="allow-vgpu" value="1"/><restriction max="255" property="number-of-vbds"/><restriction max="7" property="number-of-vifs"/></restrictions>
                          VCPUs-number ( RO): 2
                     VCPUs-utilisation (MRO): 0: 0.006; 1: 0.007
                                 other (MRO): platform-feature-xs_reset_watches: 1; platform-feature-multiprocessor-suspend: 1; has-vendor-device: 0; feature-vcpu-hotplug: 1; feature-suspend: 1; feature-reboot: 1; feature-poweroff: 1; feature-balloon: 1

We need a bit more context. What kind of backup exactly? XOA or XO sources? Which version?