Agree with Dane, unless you are extremely lucky there's almost no chance of getting a breeding pair. Even if you were to find someone willing to sell you a breeding pair then anything from £500 quid and up into the thousands wouldn't be an unsurprising price to be quoted.
While people have been succesful with just 2 zebras, for the most part people generally keep them in groups of 5 and upwards. Females for one reason or another are harder to get hold of (sexing often isn't easy even when adult either), which is one of the reasons why people aim for a group rather than just two.
At present demand totally outstrips supply and it will be that way for the forseeable future. So if your intent on buying some for the aim of breeding them and for which you'll preferably need different bloodlines then you'll need to keep a constant eye on aquarists classifieds, ebay, zebrapleco.com etc or find a couple of breeders who have stock for sale, or be prepared to pay the vastly inflated prices in the few lfs's that have them. You'll also need to be prepared to travel.
That isn't to say they are too difficult to get hold of though it just isn't quite a simple as your initial post implies.
As for the number of eggs laid in a spawn it varies but is rarely over twenty, quite low considering and this is also reflected in the price. But in all honesty you'll more than likely have many more questions that you'll need to ask before you need to be asking any questions about raising fry. Getting them to spawn in the first place is fairly difficult, after that it's easy.
