How many ceramic knives do I want?

Yes, ceramic knives are the "new thing," but that doesn't make them superior. The problem with ceramic knives is that you can never sharpen them, and, as mentioned in the comments, they may chip. Don't get a ceramic chef's knife or paring knife; the answer to your question is zero.

If you really want a ceramic knife, then buy a ceramic bread knife, although you won't be getting any extra performance for the money.

Ceramic blade mandolines, however, are great. The ceramic blade will be sharper and hold the edge longer than a steel mandoline, and since you don't sharpen a mandoline anyway, it doesn't matter that ceramic doesn't sharpen.

Note, a separate, and good, question would be "what knives do I need?" The answer is it depends, but I concur that you should not buy a "set" of knives.

Z E R O - they break, they are tough to sharpen, and the really don't hold an edge as well as people think. I personally have tried several and they are too light for me. I like a heavier blade personally.

The best knife setup I have seen (and I cooked professionally for ten years) is a good 'ol cheapo-cheapie, purchased form an Asian market (restaurant-style, white handle) and then use this sharpener frequently to keep it sharp:

http://www.accusharp.com/

I have seen people waste so much cash on costly, high carbon steel, Japanese blades and, while they are nice (and often have a superior feel), I have always done real well with the scenario described above.

They're just tools. If you're fine with your steel knives, keep 'em.

I'd just buy one and see how I liked it.

Edit: I never buy sets. I buy individual knives to fill individual needs. Soft steel carving knives, for paper thin slices. Hard steel utility knives for day-to-day chopping.

If you've got nothing, I'd head out and get some cheap high-carbon knives. (In my opinion) you only really need three knives:

1. Chefs knife for chopping
2. A 4 or 5 inch utility knife
3. A santoku, or a boning knife, depending on whether you're a vegetarian or a carnivore

That's all you need. Once you've got those, then you can figure out which ones you want. I have a nice ceramic santoku knife, which is pretty cool, but not as beloved as some of my older steel knives.

The only advantage that I'm aware of is that there are a few items that will discolor because they've been cut with a steel knife.

Lettuce comes to mind, but they also make really inexpensive serrated plastic knives that'll work for lettuce without breaking the bank. There might be other stuff out there that I haven't worked with, or it hasn't bothered me, or as a home chef not prepping food 4 hrs before service, it doesn't discolor fast enough for me to notice.

So... how many ceramic knives do you need? None. For how many knives (of other materials), see What knives are “required” for a serious home kitchen?

Maybe I am not a serious cook because, unlike other answers, I love my ceramic knives. I have three of them for casual kitchen work like peeling apples or cutting bread. I like how they freshen up the whole mood of my kitchen with their plastic look. (I bought colorful Kyocera knives from Japan.) Also they don't leave metal smell on fruits, fresh fish (for sashimi), etc. and are very easy to clean.

However, like others, I don't recommend you to buy a ceramic chef knife although there's a specially strong black one and my set has never been broken or chipped even after falling of kitchen table twice.

I kind of agree with what seems to be the general consensus: you don't need any ceramics. With that being said, here are the positives: when new they are very sharp, they hold their edge well, and do not stain certain veggies as a carbon steel knife will do. Now, the downside: THEY BREAK! easily, You cannot pry, gouge or whack with them. They will break (or at least, chip) Carrots and other colorful veggies and fruits will stain the knives. As a knife maker, I am biased toward steel blades. They will do everything a ceramic knife will do and more. A good quality steel knife will be the last one you will ever have to buy and you can get them sharpened at many places (or by yourself). If you must, get a ceramic paring knife for little stuff but in the end, a good, high carbon, stainless steel, forged chefs knife will always be the workhorse of your kitchen.

I have one Ceramic knife. a 15cm Cooks. I got it in mail. Flexed it (a little)

Cuts my cheese blocks beautifully. I've never been game to use it on anything else. It's nice and safe in it's sheath in the third drawer down.

Wustof. 20cm Cooks. 15cm Utility. cheap Parer and a coupla meatworks heavy duty blades for pumpkin. Boning etc. They do me fine thank you....Tried a mates Shun Santoku. Prefer thicker\heavier blades myself. Bloody sharp though.