In regards to the number of items you get for xp, it's my assumption that you get 1 item for 1-1000 xp, 2 for 1001-2000 xp, 3 for 2001-3000 xp, 4 for 3001-4000 xp, and then 5 for 4001+ xp (the beginning and ending number for each tier aren't exact and it could be 1-999, 1000-1999, etc.)). Who knows if the numbers are exactly correct, but from my experience that's what I've come up with.
As for my experiences, I had under 1k once (got like 600ish xp) on blm for joining in when the boss of flux 9 was just about dead by the time I got to it and I got one item total and it was a coin. A few times on blm and whm I got 3kish xp and got 4 items every time, with 2-3 of those items usually being coins (in flux 7). I never actually got 2 or 3 items in a run, but I'm assuming that you get 1 item for every 1k you earn up to a total of 5 items based on my experiences with getting 1 and 4 items. Also, since Bojack got 1.5k xp and got 2 items, that would hold true to my pattern.
As for getting 5 items, every time I've gotten over 4k xp, I've also gotten 5 items, so I'm pretty sure that's the cut off point. Sadly, on blm getting 4-6k xp is pretty common no matter how much damage you do, unless you spam cures on people, so I'm pretty used to getting low xp in WoE.