Plural nouns, like "tights", cannot be used with numbers or the article "a" ("a" is for one). If you want to count these items, you can say "a pair of", "two pairs of", "three pairs of", etc.
It's "a pair of tights". "Tights" is a British term that only has a plural form.
I have to buy a pair of tights.
I have to buy a tights.
Plural nouns, like "tights", cannot be used with numbers or the article "a" ("a" means a single thing). If you want to count these items, you can say "a pair of", "two pairs of", "three pairs of", etc.
There are five pairs of tights in the drawer.
In everyday English, however, we typically use a possessive pronoun (my, your, her, etc.)
Your tights are on the bed.
Remember, when talking about something in a general sense, we use no determiner.
Tights are often made of nylon.
Be also aware that plural nouns, such as "tights", require a plural form of a verb. For example, we say "tights are" (not "tights is").
Are your tights green?
Is your tights green?
A partial list of pieces of clothing that do not take "a":
