What's this number on my friend's Battle.net Portrait?

That's their HotS profile level (player progression).

The game adds up all 3 of their racial levels (the ones based on XP gain in multiplayer) to get their profile level, and displays that in the bottom left of their portrait. It doesn't show up if they haven't hit level 1 for any of the races, so possible values are 1-90.

Picture above is from leveling announcement back in the Beta, and still show 20 levels per race. Release version has 30 levels per race (EDIT: 35 levels per race as of Jan 21, 2014), as obvious from Rewards menu of any player profile - check out this random guy, Zealot skin reward.

The number is player progression,it is counted as (Protoss+Terran+Zerg race level).Each race can have maximum of 20 levels making the max player progression level of 60.

Official battlenet source