You can buy as many domains as you want. It is even possible to point them at the same website. However in reality this is rarely useful. You are likely to only appear under one of these domains on the search engines. If you promote your company with more than one domain people are likely to get confused. Also if you buy ten domains remember you have to renew them every couple of years, At £10 each this may not seem like much, but when it comes to renewing them it soon builds up.