Separately for your question:
reply_count in json refers to a specific reply to a post, e.g. clicking on this particular reply button below the post:
whereas replies is essentially a count of every reply from the OP (topic / first post).
Using this meta topic’s json as an example:
https://meta.discourse.org/t/why-isnt-discourse-more-frequently-recommended-as-a-community-platform/221040.json
if you notice, post number 1 has 3 direct replies even though the topic has many replies.
The gist of it is a “reply_count” is the number of replies to a post, and it doesn’t indicate how many replies a topic has.