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Don't worry that sort of thing happens to all of us.
Last time I had it happen with a Dendrobium I just put it back in the same pot, and topped up the medium with some fresh I had, because the medium was not very old anyway. If you want to take the opertunity to repot you can, but if the current medium is fine then you could just stick with that.
As to how many roots it should have... well the more the better. Do you have other rotten roots, if so it could be a sign that there is a problem. I just have a couple of roots on one of mine, but I know that those are growing and increasing in number (they rotted 12 months ago) so I'm not worried, however if they are currently rotting it could be a problem.