What does Thoreau mean by "keep your accounts on your thumbnail"?
Thoreau is making a case for simplicity; in all things, he advises, keep the number and amount of things you are involved in to a minimum (he seems to imply 10 would be a good number, as an ancient human would count on his fingers, and his toes if necessary, but no more than that).
Some interpreters take "accounts" to mean banking, business or other financial accounts, and that Thoreau is advising that we keep our business affairs small, simple, easily tracked, etc. and avoid large debts or long-term engagements. This may be a bit of an over-specification of the word "accounts" - a less specific meaning of the word would simply be "affairs", that is to say, one's personal matters. Basically, don't get involved in too many things; keep them small and few enough that you could metaphorically fit them on your thumbnail.
This interpretation, in my opinion, makes more sense in the context of the essay. Thoreau, at the point at which he makes this statement, has been speaking in very broad and somewhat repetitive terms to emphasize his point; while it's entirely possible that he's talking about bank accounts and finances, it seems more likely that he's simply offering the same point phrased differently, for the sake of variety and emphasis.