You should only ever buy one complete bike. Buying more than that is greedy, ungreen, and ostentatious.
However it is perfectly ok in the course of using it to do preventative maintenance and replace components with upgraded parts. This is a green thing to do.
Now to stay green and environmentally friendly you should not dump the old parts. Just retain them in the shed.
After a while you will have done sufficient upgrading to your original bicycle (usually after replacing the frame) to realise that you have enough parts to build another bike. This is called "recycling" and gives you great credentials as an environmental crusader.
Naturally you will wish to upgrade this "new" recycled bike because it closely resembles your original bike, and so the process starts again.
Only this time you have 2 bikes to upgrade. This requires serious dedication. Fortunately you now seem to be on first name terms with your local lbs, and if you bring biscuits, they'll even give you the odd discount.
Soon you will have 4 bikes, the lbs will have a shiny new van, then you will have 8 bikes, followed by 16, 32 etc. I believe there is a branch of mathematics devoted to the study of this phenomenon.
Unfortunately about this time you will need to find a new lbs because the original has retired to the Bahamas even though he was quite young.
Fortunately this will not be a problem, because Chain Reaction will be delivering your parts in a dedicated limousine.