WP Rest API v2 filter and display latest post with specific tag

You can actually find extensive documentation on wordpress.org.

But let’s have a look at this particular example:

https://demo.wp-api.org/wp-json/wp/v2/posts?per_page=5&tags=3

The 5 in there is the number of posts you want returned.
The 3 is the id of the tag you want to limit the posts too.

If you want to limit to a category use categories instead. But you’d have figured that out yourself after looking at the documentation, right? 😉