Structure: how many?
Structair: cia mheud?
The word tha appears in the question so we know to answer using the tha / chan eil structures in the answer.
Cia mheud … a tha … aig… ?
How many … do(es) … have?

The noun after cia mheud always takes the singular.
We can use the prepositional pronouns for aig to create questions about others.
agam | have / at me |
agad | have / at you |
aige | have / at him / it |
aice | have / at her / it |
againn | have / at us |
agaibh | have / at you |
aca | have / at them |