This is a fabulous example of manipulation. Basically, a form of argumentum ad populum, creationists found 300 'scientists' who signed this petition against evolution. Now, let's assume for the sake f argument that they all knew what the hell they were signing (And this in itself is doubt-able to an extent). They found 300 'scientists'.
There are many fields of study in the realms of science from chemistry to astronomy. Exactly how many of these people were biologists or paleontologists? In other words, how many of these people actually had a clue what evolution was about? I'm a computer geek. This doesn't mean I have a damn clue how to build a calculator. A computer is a machine. I also have no clue how to rebuild a cars engine. My vote on whether Audi has the most dependable car is utterly useless.
As for the number... They found 300 names. How many scientists are their in the world? A great many people become scientists in one form or another. I would say that about 60 percent of the planet have jobs in the scientific fields.
Real science had a rebuttal to this, and people created a new petition. They had scientists who were in favor of evolution sign this list. There was a catch, however, and only scientists named Steve (or a variation of this) were allowed to sign, thus giving it the title 'Project Steve'. As of 2011 there has been about 1170 scientists named Steve who have signed in favor of evolution.