I presume aisling-taibhse wouldn't have asked the question in the first place (11 months ago) if Duolingo hadn't rejected "how much do you have?" as an answer.
When Cé mhéad atá agat? was first added to the database, it was added as "How many do you have?". When people reported that "How much do you have?" should also be an acceptable response, it was added. This type of issue cropped up a lot more often when the course was new - at this point, most of these alternative translations have already been added to the database, so newer users don't run into this type of problem, though once Tree 2.0 is released, it will probably happen again.
The interrogatives cé = "who", cad = "what", céard = "what", conas = "how" require a direct relative clause.
The interrogatives cá = "where", cén fath = "why", cén chaoi = "how", as well as the combinations with prepositional pronouns cé/cad leis = "with what", cé/cad air = "on what" , etc. require an indirect relative clause
A direct relative clause uses the independent form of the verb (tá), an indirect relative clause uses the dependent form of the verb (bhfuil).