In English you can use 'a' or 'an' to designate singular form , so 'an elephant', 'a dress', 'a problem' will be always about a single item. However, you can't use 'a' or 'an' with every noun, since some do not have a singular form such as the word 'pants' for example. So if you want to relay information without using a number for singular, use the 'a' or 'an'.
There are more exclusions than just the word 'pants' unfortunately, and this is where it may get confusing. There are also some nouns that show quantity bigger than one which use 'a' or 'an' and the word itself designating the quantity is singular. Makes no sense? Keep reading and I will explain.
The word 'bunch' is one of them, so 'a bunch of flowers' or 'a bunch of letters' would literally mean 'a single group (bunch) in which there are many letters'.
Which leads us to the word 'bunch'. A box, a bag, a container designates a single item , yet inside that single item there are always (assumed by listener) more than one.
For plural, in American English, you can use an abstract word such as 'bunch' which will lead the listener to assume there was some quantity greater than one but the quantity is not important enough to the overall message.
It's important to note that 'bunch' has a limit, since a 'bunch of assignments' is less than 'many assignments', 'lots', or even less than 'a whole bunch of assignments', yet is less than 'a couple of assignments' or 'a few assignments' , though all designate some unknown quantity greater than one.
So if you tell someone that you won a "bunch of money in the casino", the reply is likely to be (from a friend) 'How much?', since your used a word that hides the importance of the exact quantity "bunch". So if you reply 'I won $2!', the listener will think you are making a joke.
Also the use of they/them/us/we/he/she/it automatically lets the listener infer the quantity, so 'and then they shot them' vs. 'and then he shot it' vs. ' and then they shot at us' instantly lets the listener know (a) the size of group doing the shooting (b) the size of group being shot at (c) whether the person telling the story was in any of the groups or a bystander.
The following curse/slang expression, especially in cities like New York, is used quite often (less frequently around people in power such as teachers, judges, grandparents or bosses) is 'f*ck-load' which means a lot - 'My boss gave me a fu*ck-load of work, so I can't make your poetry recital tonight'. It is best not to use this except around people who enjoy using slang themselves, unless you have a 'f*ckload of people with guns running behind you and you are screaming for everyone to hide'.
Here is something - the word 'you' can be singular or plural. If a teacher is looking at one student, while tapping another one on the shoulder and then turns around and says 'you, come with me', there is a good chance that both students will first look at each other in a confused way to decide if one, or both or the whole class should follow. This confusion is sometimes qualified by using 'you all' or 'you two' or 'just you' , but not always, which leads (though not often to confusion).
As for irregular words that are both plural and singular, there are maybe a dozen of them: deer, shrimp, sheep, fish, moose, scissors, pants.. these are both plural and singular simultaneously.