Minimum number of threads : 1 (you can set 0 but migration won't start until you set 1 or more.)
Maximum number of threads: 32
For more information, please refer to:
"Migrator for Notes to Exchange 4.14.1 – Administration Guide"
Specify Migration Run Information
"Migrator for Notes to Exchange 4.14.1 – Program Parameters Reference"
[General] section (See "ThreadCnt")