While looking at the part of speech of the noun "thanks" in an online dictionary I noticed that it was a plural noun and wondered if it could be used in singular form. Glancing at the origin it appears that the old English word "thanc" was once used as the singular form of this noun. I assume it would be appropriate at that time period to say "I have a thank!" if you felt thankful for a single thing instead of being limited to always say "I have thanks!" as we do in modern English. 

Nowadays, I believe we only use the plural form "thanks" as the noun in all cases. We can never have a single "thank". Instead we can only have "thanks"! I find this odd! It would make sense to have kept the singular form of the noun. Often we are grateful for a single action or event and it seems more accurate to have a single "thank" rather than many "thanks". 

Is there any known specific background history as to why this is?