They will send you a new card in the mail. I recently renewed my membership (it now expires in the year 2027!) and they sent me a new card.
If you lose it, they should send you a new one if you ask. I wouldn't be surprised if they now had the ability to PDF it to you, and you could print it yourself.
I'm not even sure why we need membership cards anymore. I never have a need to show or use mine, at any tourney or anywhere else.
I just renewed my USCF membership for my first time. I do have a problem: After you renewed it, will they give you another card or do you still keep the old one? And what if you lost your membership card? How is the USCF going to give you another card?