Quick one:

As I wrote in one of my previous posts, I am not a fan of running COUNT(*) on any table. I prefer these alternative methods that depend on meta data rather than running a scan on table just to get a count. No matter the size of the table, you can get the row count using meta-data view.

This post is only about COUNT_BIG(*), so:

When you need to perform COUNT(*) on tables that have more than 4 billion rows (or 2 billion if they only use positive integers [greater than 0]), you need BIGINT version of the COUNT(*).

Hope this helps,
_Sqltimes