Since the word "pants" only has a plural form, you cannot use the article "a". However, you can say "a pair of".
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.
Why Are Pants So Big (Again)? - The New York Times
Step-by-step: how to sew a pair of pants
What's up with a “pair of pants” in English? It's just one article
Trouser Trend: How to Find the Perfect Pair • BrightonTheDay
I own one pair of pants. (I hope this meme I made is actually
How should jeans fit? - Hockerty
Features Of A Pair Of Pants
Features Of A Pair Of Pants
How Should Your Dress Pants Fit Properly - Suits Expert
How to Tailor Your Jeans, Chinos and Trousers % %
The combat trouser conundrum: why would anyone wear a pair of £725
How to Make a Pair of Pants (with Pictures) - wikiHow
How to Recreate a Pant Pattern
3 Ways to Hem a Pair of Pants - wikiHow