Hello Luc,
Yes, I think that to have all the backup jobs for a specific client in specific volumes, you will need to have one pool for each client. With regard to the Label Format, If you will have more than one job for the same client, I would suggest you to not have the ${JobName} present in the label. Even if you have configured "use volume _once_ = yes", this should work for the first time, when the volume is labeled for the job. After that, any other job for this client could use this volume and the JobName information in the volume label could be inaccurate.
If you need client+job being associated always to specific volumes, you should have a pool specific for a client+job pair. I do not know your needs, but I think the first case is enough in the most cases.
Best regards,
Ana