In time they'll probably find all, but they're probably not going to count each one as a new backlink. I think too many people used forum signatures in a spammy way so they're all taken into account. I'm not sure what an actual percentage would be. I've heard Google will only count a few sitewide types of links on any site and then not put much weight into the rest, though I've seen examples where they list thousands of links from the same site in a backlink check.
Either way I wouldn't worry too much about the number of links you'll get credited for in forum signatures.
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