If you speak English you are used to words that are inflected by number. We say "John has a cat." Or "John has one cat." Then we say "Mary has many cats." Or "Mary has ten cats." Cat in one, cats in the other.
Yet suppose we met a foreign-born person who said "Mary has many cat. She has ten cat." Would we get all confused and not understand him because he didn't say 'cats'? No, we would know what he meant.
The inflection by number is not needed for understanding. Languages don't have to have this feature to function. Even in English, there are a few words without different plural forms, like 'sheep'. Do you understand when people say 'There is a sheep' or 'John bought several sheep'?
When the number of something is important, modifying words can be used, as 'a, one, many, ten' are used in the example sentences.
In Esperanto sen Fleksio, the first rule is that the suffix 'j' from standard Esperanto is not used. This means that all nouns ending in 'o' are not singular, but have a singular-or-plural meaning.
So: in Esperanto sen Fleksio, 'kato' means 'cat or cats'. So it's a bit different from the standard Esperanto 'kato' which means only 'cat', and needs to change to 'katoj' to mean 'cats'.
But what if the number is important? Suppose you are trying to translate 'There is a God.' To say 'Esti Dio.' could mean 'There is a God' or 'There are Gods'. So in this case, the number is important, so we would say 'Esti unu Dio' or perhaps 'Esti nur unu Dio'.
In the plural, one can use numbers as well: 'tri hundo' or 'dek kato' or 'dudek shafo'. If you don't know the number, there are words like 'kelka, multa, plura' that you can use. (Note we leave off the '-j' in Esperanto sen Fleksio.)
You might ask 'can't the people whose native languages don't use plural forms just study a little harder and learn how to use them?' Of course they can, and they do. But it is known that people in Asia often find learning Esperanto quite difficult, since their languages are quite different from European ones. Rick Harrison, the creator of the original Esperanto sen Fleksio, did the experiment of creating EsF in order to make Esperanto just a little easier for Asians to learn. This does not mean that we want standard Esperanto to go out of use and be replaced by EsF, by the way. Standard Esperanto is a natural literary dialect for Esperanto sen Fleksio speakers.