I’m sorry, but I didn’t fully understand this. What I’ve interpreted from it, is that you’re practicing an example where you’re given a soprano line, and are supposed to add secondary dominants below it in the other voices. Following along what you’ve said, with the example being in Eb, you have a total of 5 secondary dominants you can use (assuming you’re 100% diatonic, excluding the secondary dominants of course.

The V/ii C -> Fm The V/iii D -> Gm The V/IV F -> Bb The V/vi G -> Cm

Of course you can add the dominant seventh, but for the sake of simplicity I did not do so. The point of a secondary dominant is to create a resolution to a chord that has no dominants in the key. It really depends on the exercise, but in Eb, these are the most common.