If you go with a military style wool blanket, look for someting in 4lbs and wool blend; higher % the better- Rothco is a great brand and they've been an OE supplier for military clothing for ages.
I'm in Tampa so I'm bit warm blooded: I find that each blanket works for about 15-20 degrees so if the projected temp is going to be in the mid 50s, 1 blanket needed assuming I'm wearing a t-shirt/ shorts.
Something else worth considering is wearing a fleece with your wool blanket- like this one:
It's part of the military cold weather gear setup and is worth about 20-25 degrees. So this fleece and 1 blanket is good down to about 43-45 degrees.
FWIW, my cold weather sleeping set consists of military surplus which I feel is the best bang-for-the-buck:
- 2 wool blankets
- 2 poncho liners
- military 4pc modular sleeping bag (advertised to -40F when used with military extreme cold weather clothing system ECWCS)
- I normally sleep in the fleece and if it is below 35F, I'll throw on a balaclava- for <$10, it's too cheap to pass up.
If temps are going to be below 30s like in the 20s or teens, I would also consider getting some gortex gloves.