First, capture, tag, and release a number of US senators. 15 or so should work. Then, reset your traps and capture some more. Count how many of the second capture set were tagged in the first group. This tells you what proportion of the population of Senators you captured the first time, and by dividing the number of senators you tagged by this proportion you can estimate the total population of senators. Divide that number by two to obtain the number of states.