An intention is a condition for the validity of the prayer. Thus at an absolute minimum, you should intend that it is  a makeup and,  which prayer it is (i.e. Dhuhr, `Asr, Maghrib etc.)
Practically, it would be simpler to intend that the specific (e.g. Dhuhr) makeup prayer is the most recent one you missed. This is easier to sustain than mentioning a specific number (e.g. 57th) which is likely to become very cumbersome if you have a large number of prayers to make up.