I don't know how often GR checks in here, but, basically, the answer is that Cé mhéad and Cá mhéad mean the same thing, and both are correct, so just take your pick.
How many is Cé/Cá mhéad followed by the
nominative singular form of the noun, so you're really saying How many dog? or How many tree?
How much is Cé/Cá mhead followed by the
genitive singular form of the noun, so you're really saying How much of flour? or How much of time?
cé mhéad crann = cá mhéad crann = how many trees (Crann is masculine, first declension, so crann is nom. sing.)
cé mhéad plúir = cá mhéad plúir = how much flour (Plúr is also masculine, first declension, so plúir is gen. sing.)
Here's a blog about 'How many,' which seems to have been the issue in the original question: https://blogs.transparent.com/irish/ce-mhead-realta-ce-mhead-riabh/
Please read the other comments. Irish doesn't use plurals after numbers, and it doesn't use a plural after cé mhéad or cúpla, etc
It is. Does Duolingo tell you anywhere that you always use the singular after cé/cá mhéad? It looks as though this has been pointed out on this page a couple of times already, and I still don't know whether Duolingo actually teaches/tells you that or expects you to know it by magic.