depends on the extension, and to some extent on the topic, CPC, length, etc.. For a .com, 1,000 is pretty good (I like to see a minimum of 250 for a .com). As the extension gets worse, the number of exact matches needs to go up. Before considering a .org, I like to see at least 2400, a .net, 4000+, a .us 10K+. It is still only one variable in determining value, and you shouldn't use that factor only.