Supposing an even distribution of 180M visits per day and 1 requests per visit:
125,000 visitors per minute
125,000 http request per minute
2,083 requests per second
That should be doable on a large vps of some sort, even for wordpress.Now the question is, how complex is the workflow? Certainly a visit will consist of more than one request. Also, visitor frequency will most likely peak during certain periods of the day/week.
http://reviewsignal.com/blog/2016/09/14/500month-enterprise-...
if it's static cache hits, tons of companies can do 5000/second without issue.
If you're talking about 100,000+ visitors, than you'd probably want to get some good advertising and update your RAM.
Focus now on getting lots of uniques.
What Wordpress may not scale to support easily is the complexity of features that a site may have accumulated by the time it gets to 180M users.
The few 10000 users companies then need to worry about getting to the million users stage.
And so on.