There are some countable words in English which have no plural form. Or you could say the plural form is the same as the singular form. For example, deer and sheep. 1 deer, 2 deer, 1 sheep, 2 sheep.
Now of course, you do hear native English speakers who accept and speak using incorrect grammar. So, just because some natives will tell you that it is OK or acceptable, you do not have to start using incorrect grammar. One widely accepted incorrect usage of grammar is when people put an 's' on 'beer.' They'll say, "I drank two beers last night." What they should say is, "I drank two bottles of beer," or two cans of beer, or two mugs of beer. Beer is a liquid. Liquids have neither shape nor size. You must put them into something in order to determine just how much you're talking about. Or you can be more exact and use a unit of measurement for liquids, such as ounces.
Another one is 'content.' Now, there are two words, 'content' and 'contents.' Contrary to some dictionaries, I argue that 'contents' is not the plural form of 'content.' I use the simple reasoning that we do not put a number in front of the word to count. It is uncountable. We do not say, "I have six contents in my purse." It is not a counter. Instead you would say, "I have six items" or "six things in my purse." You can say, "Let me see the contents of your purse," or you can say, "Let me see the content of your purse." There is no difference that I am aware of, however, the one with 's' sounds better to me in that sentence.
There are lots of things which cannot be explained by grammar rules. We call them "exceptions to the rules" in English. One thing which I cannot explain, is when a word uses the singular form or the plural form after the word 'no.' Such as,
- "I have no clue what you are talking about."
- "We have no clues in this investigation."
- "I had no idea what you were doing."
- "I had no ideas for the project."
- "I have no idea."
- "I haven't a clue."
- "I have no good ideas."
- "I have no clues."
- "I have no excuse for my actions."
- "I make no excuses for my actions."
- "There is no excuse for this kind of behavior."
- "There are no excuses for this kind of behavior."