I am a tad confused as to how "koni geltī" is the singular "that helmet" and not the plural "those helmets."
Why not 'Kona geltī emā?' Shouldn't the question as put be translated into 'Do you have those helmets?'
Gelte does not decline like ābra: geltī, with long i, is accusative plural and singular. But it has to be singular here, because koni does decline like ābra, and therefore koni can only be acc sing.
"Koni geltī emā?" = "Do you have that helmet?"
"Konī geltī emā?" = "Do you have those helmets?"