The taxes are no harder with a dozen treasury notes than with one. You get one combined statement with the total of all your interest payments, which is all you need to do the taxes.

The only advantage to getting a few, larger notes instead of many smaller ones is if you're keeping track of everything manually, and have to enter all the small interest transactions by hand.

I'd be inclined to go with buying a note everytime you have some extra cash. For one thing, you're less likely to get hung up on whether you're getting the best rate. (Worrying about that sort of thing can drive you crazy.) Also, if you have just a few notes, your income can change a lot each time one of them matures and you reinvest the money (getting whatever the interest rate is then). With many smaller notes, the effect of falling interest rates would only hit your income gradually, giving you time to adjust.