How many anchor text variations should be in a strong campaign?
Try and think like an end user who is linking to your site, How would they do the anchor text ? Thats what google will be trying to spot. The biggest problem I have sometimes is I am too close to my project so find this more difficult than it should be. Its always tempting to zoom in on your keyword when you find a great site that you can post a link from but diversity is really the key. I cant really answer your question in numbers and proven facts except to say mix it up as much as possible including some click here, read more etc .