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.
Since the word "pants" only has a plural form, you cannot use the article "a" or numbers with it. However, you can say "a pair of pants", "two pairs of pants", etc.