As a best practice and the recommended way cluster should have minimum three nodes.
Primary with Two Secondary Members (P-S-S)
These deployments provide two complete copies of the data set at all times in addition to the primary.
These replica sets provide additional fault tolerance and high availability. If the primary is unavailable,
the replica set elects a secondary to be primary and continues normal operation.
The old primary re-joins the set when available.
if circumstances (such as cost) prohibit adding a third data bearing member, two data-bearing members and an arbiter (Primary-Secondary-Arbiter)

Primary with a Secondary and an Arbiter (P-S-A)
Since the arbiter does not hold a copy of the data, these deployments provides only one complete copy of the data.
Arbiters require fewer resources, but at the expense of more limited redundancy and fault tolerance.
However, a deployment with a primary, secondary, and an arbiter ensures that a replica set remains available if the primary or the secondary is unavailable.
If the primary is unavailable, the replica set will elect the secondary to be primary.