1. Source and drain regions have contacts, not vias.
2. For submicron processes generally a minimum of 2 contacts (or vias) are required for each connection, for reliability reason. There are, however, exceptions for some small Standard Cells (e.g. a low drive inverter), if a second contact|via would require an area extension of the cell. Here reliability is traded for area saving.
I don't think this is a universal rule - fewer number of contacts in drain than in source of a MOSFET.
Typically, the number of contacts is equal.
I can think of several different reasons why making two contacts in source and one in drain would make a sense:
1. two-finger transistor with shared source - so that source carries a double current than each of the drains.
2. in some circuits, people try to minimize gate-drain capacitance, so having less number of contacts in drian leads to smaller gate-drain capacitance (if it is dominated by gate-to-contact capacitance).
This is an important background information (that this question is from an interview, and not from real life).
At interviews, people often ask questions that are ill posed, or don't have a simple answer, or don't have a known answer at all - to see the candidate's response, reasoning, attitude, way of thinking...