The number is surely not so large. Just as you have rightly equated a series of pitch differences starting on C with the same series starting on any other note, you should equate a series of pitch differences comprising only semibreves as being the same as the those pitch differences comprising only crochets. And is a 2 note melody, say C semibreve C minim, different to C minim C crochet, or C quaver C semiquaver, or even C demi-semiquaver C hemi-demi-semiquaver.

Perhaps we will run out of melodies after all