The minimum total number of agents (busy + idle) that should always be running for this configuration. The plugin will proactively launch agents to maintain this count.

When the retention strategy would terminate an idle agent, termination is blocked if it would bring the total count below this minimum.

Example: With minimumNumberOfInstances=3 and minimumNumberOfSpareInstances=1:

• build#1 starts: agent-1 busy, agent-2 idle, agent-3 idle
• build#2 starts: agent-1 busy, agent-2 busy, agent-3 idle
• build#3 starts: agent-1 busy, agent-2 busy, agent-3 busy, agent-4 launched (to maintain 1 spare)

Set to 0 to disable (default).