How much deposit should I ask for?

It is up to you how much deposit you want to ask for but by law it should not be higher than 10% of the purchase price. A deposit does not have to be paid under the contract, however it is in your interest to have it paid as it is your security under the contract. If the buyer signed the contract, paid the deposit, but is otherwise unable or unwilling to complete afterwards, you may be able to claim the deposit paid.
You should discuss with your real estate agent on what is the appropriate amount to ask for on a case by case basis. Remember that whilst the deposit is a consideration, it is not the only consideration on whether your proposed buyer is genuine about their offer to you.