How many island are there on minecraft skyblock?

It depends on which version of Skyblock you have. The first version had two islands; the starting island, which had little more than an ice block and a bucket of lava, and a Nether island, which has items needed for various challenges set by the map's creator. The newer versions have another island in the overworld. This one is made of sand (make a platform below it before you do anything with it!) and has a few items (and another ice block, if I remember correctly) for the challenges.

That depends on the map, many skyblock maps are different than others.

The first island is usually a short L shape, three blocks wide and three blocks deep. Most are entirely dirt blocks, but some will have an iron ore or coal ore block hidden in the center.

The first island usually includes the following:
Dirt (with grass on the top)
A tree or sapling (trees are more traditional)
A chest with:
A bucket of water (some use a block of ice instead)
A bucket of lava

Also depending on the version the first island chest might contain:
a bone, some string, a reed, some seeds, a torch, and some mushrooms. (Since a reed is more or less required for some things, there *will* be reed somewhere, if not on the first island then on the second.)

The second island is almost always made out of sand, with a cactus growing on it. Depending on whether you plan on give players rewards, it is not unusual to find a chest with three diamonds in it, iron, and/or flint. (So the player can think about making a nether portal). If there was no reed on the first island, there will be reed on the second.

The Nether island will have Netherrack and Soulsand, not unusually with several mushroom on it and glowstone.

At one of the secondary islands, there's always another source of water, so an infinite water-source can be created.