My friends and I are having an argument on how many diamonds a Diamond Ore can drop (on avg. or maximum) without any fortune levels on a pickaxe. My friend says there will always drop only one, but I'm fairly certain that's not true. Tested in singleplayer, I've had a few times where diamond ores have dropped 2 diamonds while having absolutely no modifications installed.
I hate to break it to you, but your friend is correct here. Without any sort of enchantments or mods, diamond ore only drops 1 diamond. There's a high likelihood that you might have simply miscounted the number of ores you mined in single player.
Note that the average number of diamonds with a Fortune enchantment are 1.33, 1.75, and 2.2 for Fortune I, II, and III respectively.
Well... Acording to the wiki :
Diamond ore mined using an iron or diamond pickaxe will drop a single diamond.