you can bulk restore by calling 'pct restore' / 'qmrestore' (or the corresponding API endpoints, or if you do it manually, also the restore feature in the GUI). there is nothing that forces you to only restore backups one at a time, other than possible contention on some locks (e.g., allocating a new disk may require obtaining a storage-wide lock for a short amount of time to prevent naming conflicts). I recommend staggering and batching the restores, you likely won't benefit from having more than a few restores running in parallel if the backup source and the storage target are identical across the jobs.