Chris,

If that is (as I suspect) 4" x 4", you need 9 for each square foot, thus 90 overall - if they fit the region you wish to tile perfectly. Usually they don't.

If you need to cut tiles, the number you need will depend on how you cut them. Some methods let you use both pieces, others may not.

Wooden tiles can be sawn with little waste by anybody who has any experience using a saw. So can sandstone tiles - a hacksaw, used almost flat, works very well.

"Snapping" ceramic or hard stone tiles involves no waste, but except in the hands of an expert will result in a moderate amount of wastage, where one or both sides break. Cutting results in a kerf of about 1/4" that is reduced to dust, but both bits are usable. You can cut tiles with a "wet" tile saw (easy to rent, or I've seen them on sale for as little as $60). An angle grinder with a diamond wheel also works and is cheap - "Diamond-Tooth Gertie" has helped me out on various projects.

You should probably do a rather large scale drawing on squared paper to figure out how many tiles you will actually need. Then add another 10% or so for breakage, cutting errors, and in case you ever need to make repairs [dropping a heavy hard object, say]

-RD