Learn Icelandic: Við sjáum mikið í einni ferð.

The number "one" (einn) changes its ending to match the gender, case, and number of the noun it describes. Here, it describes ferð (trip), which is a feminine noun. Because the preposition í puts the noun into the dative case in this context, the number must also be in the dative feminine form, which is einni.

Why does the preposition í trigger the dative case here? Doesn't it sometimes take the accusative?

Yes, í can take either the accusative or the dative. It takes the accusative when showing direct movement or motion into a place. It takes the dative when showing a static location, or being situated within an event or period of time. Since the seeing happens during the trip, rather than moving into it, í takes the dative.

Why does í translate to "on" here, rather than "in"?

Prepositions rarely map perfectly between languages. While í most commonly translates to "in", Icelandic conventionally uses í for trips and journeys (í einni ferð). English just happens to use "on" for trips, so we translate the underlying concept rather than the literal word.

Could I use margt instead of mikið to say "we see a lot"?