Some sites that gather achievement data can provide some idea on this, however. I looked at World of Wargraphics site recently, and it notes that 5.4% of level 90 characters on the armory have the Black Prince: Chapter 4 achievement. Given that it records those same characters as 80% having the Dynamic Duo, 60% Terrific Trio, and 30% Quintissential Quintet, it is obvious that the site has no way of telling which character on an account has earned the achievement, so it marks all level 90s as having that achievement. Let's say there is an average of 3 level 90s per account (probably low given the data, but I'm trying to compensate by going highest probable number as well as account for accounts that have multiple characters with the epic cloak)
That makes about 1.8% of all players with a level 90 currently have the cloak on at least one character. Less that 1 in 50, many of who will get the legendary in the first week or two of 5.4. It's a fairly high number when you think of overall population, but that's the absolute upper limit - and given that numbers vastly decrease between each tier of the questline (44% marked as completed Chapter 1, 16% chapter 2, 5.8% Chapter 3, 5.4% Chapter 4), I don't think there's going to be too many additional people getting their cloaks once the first month is past - mostly just getting cloaks for alts who won't increase the data numbers at all.