The leaked Best Buy document has been found to mention that on launch day Best Buy will only have 15 Apple iPads available per store (those stores that sell it) to buy. That makes it five of each device as in 5 16GBN models, 5 32GB models and 5 64GB models.
None will be made available on the website to buy in the beginning and it is believed that once those 15 Apple iPads have sold that the next batch wont arrive till April 11.
This information all relies on the accuracy of the leaked documents though, but looking at the Apple site it does mention that all pre-orders that are made now wont be honoured till April 12 which indicates a lot of the Apple iPads have already been purchased.
There's only a few more days now till launch, so if you haven't pre-ordered already and are desperate to get your hands on an iPad this weekend then start queueing early, perhaps the night before to get one that day. Of course, we don't really know how many iPads are going on sale in the Apple stores and there might be no problems with you picking one up this weekend, but based on the second batch shipping April 12 I think a few will be disappointed.