How many concerete blocks needed to build fence around two acres of land.

How tall a fence?

A square acre is 208.7 feet, so to go around is @ 835 feet. Or 10020 inches
A concrete block is @ 16 inches long.

So you will need @ 626 to make one layer around.

Chuck is correct for a square parcel of land, one acre in size. Two acres (square shape only) would be about 295 feet per side and would require about 908 blocks. However, if you were to have a rectangular 2 acre parcel, just say for example, approx. 871 feet ft by 100 ft, the perimeter would be 1,942 feet, requiring almost 1,500 blocks for one layer. If the parcel is not square, you need to look at the plat or deed, to get the actual distances or measure the actual distances on the ground.

Good catch, I was figuring one acre, not two, so my number had to be doubled.

And the other question is, how tall a fence, 3 foot, 4 foot, 6 foot.

And I hope I was not sucked into a home work question

Actually your number would increase by about 50% from 626 to 908 for a square 2 acre lot.
Probably would have had the info you just requested if it was homework--I hope.

Next would a poured concrete wall be a better choice ? What type of foundation do they plan on using.

Poured wall around a lot that big? $$$$. What are you trying to contain?