How many Sentinel instances are recommended at minimum?

The minimum recommended is three Sentinel instances.

#Why exactly three:

1. Quorum and voting. Sentinel makes decisions (e.g. about a failover) only by majority vote.

• With two instances, agreement could end up 1:1, and the system would "hang".
• With three, a majority (2 of 3) can always be reached.

2. Fault tolerance. If one Sentinel goes down, the other two can keep monitoring and carry out a failover. With two instances, one failing already makes the system non-functional.
3. Detection accuracy. Three Sentinels help avoid false positives (if one node gets it wrong, the rest can overrule it).

#Summary:

Minimum: 3 Sentinel processes, Optimal: an odd number (3, 5, 7), so a majority vote can always be reached.