How many items can you make with Fabricate

By RAW there is no way to answer this for sure, magic missile does say it generates multiple missiles but this seems to be flavour rather than mechanical.

The key part I'm relying on in this answer is in the description of toppling spell: your force spell [snip] If the target takes damage

This clearly says that if the spell deals damage to the target then you get a trip attempt. It does not say anything about damaging the target multiple times, it just says that if the spell deals damage.

Based on this part of the rules the way I've been using the spell is to say that you get a trip attempt against each target struck by the spell that you deal damage to no matter how many missiles you hit that target with. If you hit three targets you get three trip attempts (one on each). If you hit one target three times you get a single trip attempt.

I'm aware of the sneak attack/rays FAQ but it's speaking about a completely different ability and a completely different style of spell so I do not see how it applies.

The full description of the Fabricate spell:

You convert raw materials into products of the same material. For example, you can fabricate a wooden bridge from a clump of trees, a rope from a patch of hemp, and clothes from flax or wool.

Choose raw materials that you can see within range. You can fabricate a Large or smaller object (contained within a 10-foot cube, or eight connected 5-foot cubes), given a sufficient quantity of raw material. If you are working with metal, stone, or another mineral substance, however, the fabricated object can be no larger than Medium (contained within a single 5-foot cube). The quality of objects made by the spell is commensurate with the quality of the raw materials.

Creatures or magic items can't be created or transmuted by this spell. You also can't use it to create items that ordinarily require a high degree of craftsmanship, such as jewelry, weapons, glass, or armor, unless you have proficiency with the type of artisan's tools used to craft such objects.

These are all the limitations of the spell.

Walls are objects (like most parts of the environment that aren't creatures), so a part of a wall or a piece of flooring (e.g. a wooden plank) would be considered an object as well. Depending on the size of the chunk of wall, the material it's made out of, and the level of craftsmanship making such a wall piece would require, Fabricate might be able to make it.

In your particular example, it's a stone wall, and thus you would only be able to make a Medium-size piece of wall (that fits within a single 5-foot cube) with one casting of the spell. Your DM would also need to rule that it doesn't require a high degree of craftsmanship to make. And of course you'll need the stone in some form (whether in rubble or from a boulder or something), since that's the raw material that is being magically converted into the new form.

Also, keep in mind that nothing about the Fabricate spell attaches the object you create to anything else. Even if you meet the above conditions, you'd need some way to place the chunk of new wall you've made into the hole in the wall, and to secure it there.