WoW:API GetNumBattlefieldStats

Appears to return the number of columns in the battleground/field scoreboard, other than the common ones (Killing Blows, Kills, Deaths, Bonus Honour):

local x = GetNumBattlefieldStats();

Which, currently, is 2 for Warsong Gulch and 7 for Alterac Valley

Warsong Gulch

• Flags Captured
• Flags Returned

Alterac Valley

• Graveyards Assaulted
• Graveyards Defended
• Towers Assaulted
• Towers Defended
• Mines Captured
• Leaders Killed
• Secondary Objectives