Clarification regarding a statement in "The Selfish Gene" by Richard Dawkins
"Impact on the world" is just his writing style.  Dawkins means "evolutionary significance".  For example, all the intermediate forms leading up to the modern horse have been evolutionarily significant because they contributed to survival of the lines of descent that are still present.  Any tiny evolutionary branch that dies out would have little significance or impact.

In the first part of your second question, the answer is again Dawkins' writing style plus the fact that he's writing for popular consumption.  And, his own thoughts have a way of evolving just as they should; so we shouldn't expect perfect consistency between what is said between sequentially written books.  The Blind Watchmaker was written to convince people that evolution can produce complex organisms without any pre-existing goal to do so, using a relatively simple process.  The Selfish Gene was written to promote his idea that the unit of evolution is the gene. (Note that his idea is not necessarily correct.)

In the second part of your second question you ask:Why must there be many copies of entities that are being selected for evolution to be possible?.  The answer to this is that it is NOT necessary.  It's simply a much more easily implemented - and generally better- evolutionary strategy.  If there is only one organism, and selection means killing it if it is unable to adapt to a given environmental change, then failure to adapt puts an end to it and all its possibilities.  If, on the other hand, that one organism produces a series of single copies (offspring) and each of them has some random genetic variation that's subjected to selection, there's a chance that some will make it past the selection process.  That's equivalent to the one organism producing a lot of offspring at the same time, each with its own genetic differences.

This is a slightly SiFi perspective on the above, but: In general, it's a lot safer to test a disposable copy of oneself against a new danger, than to test oneself directly.  If the copy passes the test, it's safe to face the danger directly.  If the copy fails, then you can test a slightly modified version of yourself against the danger.  If that modified copy passes, then if you're able to do so, you could make the same modifications to yourself and go face the danger.  However, there are now two identical new modified copies of you.  One of you is superfluous, so you flip a coin to decide which survives.

In the SiFi scenario, we have one organism evolving safely up one possible branch of an evolutionary tree.  But if that organism reaches a dead end from which there is no escape, its branch dies.  On the other hand if all the other branches are allowed to continue (that is, if you let both copies survive and evolve independently), there's a far better chance at least one branch will still be growing and branching far in the future.  

I personally think Dawkins "Selfish Gene" idea fails when we try to fit it into that picture.