Posts are numbered sequentially, but there are a number of different kinds of posts:
So a question having number 1000000 doesn't mean that one million questions have been asked.
User IDs (numbers) are also issued sequentially (but separately from posts). Since SE employees can know exactly when a site opens, they can often nab earlier numbers, though non-employees have been known to grab very early IDs. Of some note, Jeff Atwood (one of the founders of Stack Exchange, and fairly active throughout the network in the early years) only has ID 153, one more than Grigory M, who, as far as I am aware, has had no affiliation with Stack Exchange.
There almost certainly was a user 1, but that account would have been deleted before deletions were logged (so even site moderators cannot see any information whatsoever about the user). Note, too, that there is a user -1.