Were there other permanent settlements with over 10,000 people, built between 500 BCE and 1000 CE? The only other cities I know in Celtic-dominated places were built by Romance- or Germanic-speakers.
Where did you read that Toledo was so large? The little bit I know about that period would put 10,000 as the absolute upper end of Celtic settlements.
Alesia, for example, was estimated by Caesar to have 80k inhabitants, whereas the real number is likely lower by perhaps still over 10k. I'm aware p.d. Paris has been settled continuously since the Bronze Age, but I'm struggling to find population estimates at the turn of the millennium.
Singidunum (today's Belgrade, capital of Serbia) was built by a Celtic tribe, the Scordisci and during the Roman rule almost definitely had more than 10,000 inhabitants as it was the seat of Legio IV Flavia Felix.