Learn Spanish (Spain): Estimamos la asistencia en dos mil y pico personas. - We estimate the attendance at two thousand and something people.

The phrase y pico (meaning "and a bit" or "and something") is invariable. It never changes its form to match the gender or number of the noun it modifies, so it stays y pico even before the feminine plural noun personas.

Does dos mil y pico mean just a few extra people, or could it be hundreds more?

Y pico just means "some unknown amount more". With small numbers, it usually means a fraction or a few units. But with a large number like dos mil, y pico scales up and implies a proportionally larger unknown amount—potentially up to a few hundred extra (e.g., 2,150 or 2,400), usually stopping before the next major milestone like two and a half thousand.

Why do we use the preposition en before the number here?

The en is actually connected to the verb estimar (to estimate). In Spanish, when you estimate, value, or calculate something at a certain amount, you use the structure estimar [algo] en [cantidad].

I thought asistencia meant "assistance" or "help". Does it mean "attendance" here?