And there isn't a size 10. They go from 12, the finest, then 8 and 5.
To help you remember, perhaps you can visualise spinners in the past. They had a specific length measuring stick (Different for Europe compared to the UK, I believe). If they weighed out a pound of thread and wrapped it around the measuring stick, if it was finer thread, it would go round more times than if it was thicker thread. This is a simplified version.
So size 12 Perle would go round more times for its given weight than size 5. Hope that helps, rather than confusing you.