It will depend on the context.
You would say of a shop ice cream , "Leurs glaces sont excellentes" you cannot use the singular here.
You will say for instance:
J'ai acheté une glace à la pistache chez Ben & Jerry's car leurs glaces sont excellentes = I bought a pistachio ice cream at Ben & Jerry's because their ice cream is excellent
If you talk about ice cream in general it will be mostly in the plural.
N.B. Let's not forget that 'glace' can also describe 'frozen water' as in-
une sculpture de glace = an ice sculpture
patiner sur la glace = to ice-skate
in this case 'glace' will be singular.
Hope this helps!
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