'Banister' (more commonly spelt with one n, but the double is also an option) actually has a number of meanings. It can refer to an individual upright spindle (also known as a baluster). Alternatively, it can refer to the whole structure: a line of spindles, the newel posts at the top, bottom and turns, and the handrail. It can also refer just to the handrail. So you could say either "This staircase has a banister" (referring to the whole structure as one unit) or "This staircase has banisters" (referring to the line of spindles) , and the meaning would be pretty well the same. As you can see, this word offers us a number of options : bannister, bannisters, banister or banisters, used to mean a variety of things. Take your pick!

Hello! in this case the difference is that "bannister" is the singular form of the word, and "bannisters" is the plural form. So, your staircase could have one bannister, or two bannisters. I hope this helps!

it's difference between when to use singular noun and when to use plural noun.

In what case?