seconds for a second helping

[Fujibei]

I understand "seconds" means "a second helping." But why in the plural form?
Another question is if "thirds" "fourths" etc. are also used to mean "a third helping" "a fourth helping" etc.

[5jj]

I understand "seconds" means "a second helping." But why in the plural form? That's just the way it is. Note that this is normally used only in informal speech
Another question is if "thirds" "fourths" etc. are also used to mean "a third helping" "a fourth helping" etc. No

5