The Final Reflection, which was just cited in another thread as the best Trek novel ever, was the very first novel to have a number displayed on the cover (#16; the earlier books were retroactively numbered on the backlist and in reprints).
Yep. Although numbering seemed like a great idea at the sixteenth MMPB, Pocket found - by ST IV - that a quirk of retro-numbering the novels was that even TMP, ST II and ST III had to be kept "in print" so that new readers of ST tie-ins could "complete" their numbered collections, with the result that TMP set some kind of a record for the number of times a novelization was reprinted, long after the movie itself had completed its theatrical release. Thus ST IV received no numbering, and in UK, the very next novel, "Chain of Attack" started a new set of numbering for the UK Titan collectors.
Numbering supposedly encourages collectors to "get them all" even if a title doesn't seem to appeal. Numbers
can be an aid to shelving in production order by series (eg. it was useful when I moved house) - unless one shelves by author surname or timeline order.
It's essentially a market gimmick that starts off well but loses impact. Playmates action figures took great delight in individually numbering ST toys and, although it caused some initial excitement, some fans were distressed that legs were being numbered separate to the figures - and then fitted to random Starfleet male figures - so even "low numbers" often meant nothing as to production runs. And the really, really low numbers were often given away, and genuine buyers had no way to find them.
But numbering the covers of ST books was no guide to quality. and a nuisance for things like "The Lost Years Saga", which get scattered across my shelves.