You can use an apostrophe to show plurals of letters, words and numbers such as:

• There are now two 9's in his phone number.
• You must learn your abc's!
• Too many if's and but's in this scenario ...
• Did you know how many o's there are in Woolloomoolloo?

There are many differing rules of apostrophe use.

If you search the internet you will find that apostrophes with numbers is now considered optional in the USA and UK. Language changes and grows, and therefore rules may change.

One grammar book in the USA might not agree with another. The fact that you're willing to investigate and learn is what's key. Enjoy learning!