Let’s Encrypt doesn’t impose any traffic limit for your site (but if you use some cheap service to make the site you may need to check with them if they impose such a limit or if things would just be a bit slower if there are a lot of visitors)
Let’s Encrypt provides Certificates, which work not so very differently from ID documents you might have from a government, like a passport, or a photo ID document, or a driving license - except with mathematics instead of pieces of paper. Your site shows visiting web browsers the Certificate to prove “I’m example.com, see I have this document and I can prove it’s mine”. Just like the government doesn’t need to send a guy over to agree “Yes this is real photo ID” when you use it to prove you’re allowed to buy vodka or whatever, Let’s Encrypt aren’t involved directly when your site shows the Certificate to visiting web browsers, this can happen as often as you like without limit.
The limits Let’s Encrypt talk about are for making new certificates, you may not need to do this work at all if it’s included in the cost of your web site, if you are making everything totally by hand and need to obtain the certificates, you should make sure not to ask for lots of them because of these limits. But probably you are making a web site with somebody else’s tools and those will obey the limits.