The amount of cash that is necessary depends on a number of items. Generally speaking, though, you will need to supply:
Earnest Money: The deposit that is supplied when you make an offer on the house
Down Payment: A percentage of the cost of the home that is due at settlement
Closing Costs: Costs associated with processing paperwork to purchase or refinance a house
Look at it from a lender's perspective: If you put a large downpayment into a home, you aren't likely going to default. With this lower risk comes better terms on your mortgage. While you may need no cash to purchase a home, having cash will allow you to secure a lower rate on your mortgage.
If you are in a situation where you would rather save your money then lock it into a home, there is always the possibility of asking for a seller concession and try to get approved for 100% mortgage financing.
While this it is possible in today's lending climate to buy a home with literally no money down, it behooves the borrower to at least expect and prepare for some minimal expenses. Look at it this way, if you can buy a home for as little as first and last months deposit for a rental home, you are way ahead.
In some cases it is even possible to purchase a home with only slightest, or in some instances, even no earnest money deposit. Combined with seller concessions and/or no closing cost loans, it is possible to get into a home with actually no out of pocket expenses.
You must be prepared to come up with the closing costs plus 1 years worth of homeowners insurance upfront, pay for any application fees, appraisal fees and inspection fees.
Even though you're under the impression that all your closing costs will be paid due to seller concessions, sometimes lenders ask for more than what your loan officer expects, such as 6 months tax reserves, when normally it would be 3 months, etc. Don't be surprised if you have to supply a pittance at the closing table.
For homebuyers in California, I offer a unique service. If you choose to use me to represent you as you buyer's agent in addition to securing your financing, I can usually credit several thousand dollars towards your closing costs.
This, combined with 100% financing means you can buy a home with almost no cash out of your pocket. Call me for further details.