This is one way, but it doesn't look very good: "One, two, three, . . ."--as he counted past ten, his hands sprouted more fingers--"thirteen, fourteen."
I would have spaced after "three" and used only three periods because the sentence is incomplete. A fourth period would be the one to complete the sentence, and it'd be the first in the series, with no space before it.
Whatever punctuation you use, it might go to an editor who has different preferences and will change it.
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