How do I add the missing pieces to a set?

If your set reads, say, 4 of 6, there are two reasons and two fixes.

You acquired the rest later

If the catalog Set gained members after you added your copy, or you simply want to top it up to the full member list, open the set in My Sets and use Add missing pieces. That creates the absent member Pieces for you, linked to the set, so completion jumps to the full count.

You already own them as loose pieces

If you logged those cars individually before adding the set, they're sitting in your collection as loose pieces. Promote them into the set instead of making duplicates — see What are "loose pieces" and how do I promote them into a set?

For how the count is figured, see How does set completion work?