From the outset I would stress that this isn't usually a trick question - firms know that you will have applied for another of other firms. Even if lawyers/recruiters think that 'their' firm is the best, they aren't delusional and know that absolutely everyone in this process will have considered a number of different firms.If asked in an interview what your strategy has been when applying for vacation schemes, what kind of answer are they looking for? Focusing on the practice area strengths, international nature, and the like?
I usually approach this question by referring to some key characteristics that I looked for when deciding which firms to apply to. So you might say for example that you applied to only US firms because of the small intake and US clients, or that you applied to firms that have an established disputes practice because that's something which interests you (NB don't refer to a very niche practice area which would only be a small part of a TC at most), or that you applied to firms with an emphasis on international work/secondments because that interests you...etc. Just make sure that the criteria that you cite as a factor in determining where you applied (a) clearly encompasses the firm where you are having the interview; and (b) isn't an arbitrary measure that doesn't display a great deal of thought (for example, saying that NQ salaries were your key consideration when making applications is unlikely to come across well).
I think as long as you can show that you used these kinds of coherent factors to think carefully about where to apply (instead than randomly shooting off applications) then you are on the right track. There's no harm in giving less rather than more here...if you applied to (say) 20+ firms then it may be hard to find something which links all of them, which is fine, but I don't think that alluding to this would be the response that the interviewers are looking/hoping for.
Naturally, if you mention something as an important factor in determining where to apply, then be prepared to explain briefly in your answer why that is an important factor for you and/or answer a follow-up question about that factor's importance. (For example, if you say that a large trainee intake is important to you, then you are likely to be pressed on why that is important to you.)