Umm... I seriously doubt it is a snatch block you want.
The snatch block is a great block for changing the position of things line spinnaker
sheets, but not something I'd generally recommend for hoisting dinghies
or going up the mast. A snatch block is really useful for altering the lead of a line
without requiring the line
be re-reeved, since a snatch block can be placed on a line
without requiring the bitter end be passed through it—through use of a swiveling or hinged cheek plate.
A snatch block looks like this:
What you're probably thinking of is a block-and-tackle, which would be far more useful for hoisting dinghies
or for pulling oneself aloft. As for how many sheaves you'd need on either end, it depends on how long a rope you have and how much mechanical leverage you're looking to get.
The greater the mechanical advantage, the longer a rope you will need
—a four-to-one block and tackle needs a bit more than four times the length of the greatest height it will be used on, a six-to-one requires SIX times the height. For a 35' mast, a 4:1 would require 150' of line, a 6:1 225' or so.
You can buy a block and tackle in many places. A good source for a 4:1 block and tackle would be to buy a Garhauer Boom Vang setup.
If you're going to use a block and tackle to lift yourself aloft... I'd second the precautions that DavidPM has pointed out. I'd also point out that you'll probably want to have a bag, bucket or pouch attached to the bosun's chair or harness that you can tail the block-and-tackle line into. If you do not do this, and merely let the line fall to the deck—you may find that when you want to come down, that the line is snagged on the deck hardware and that you are effectively trapped aloft.
Using the proper terminology would really make communicating much easier.