• reshardingCoordinatorServiceMaxThreadCount - defaults to 3, minimum of 1, and maximum of 256.
• reshardingDonorServiceMaxThreadCount - defaults to 2, minimum of 1, and maximum of 256.
• reshardingRecipientServiceMaxThreadCount - defaults to 8 (or 16 if SERVER-55330 is completed first), minimum of 1, and maximum of 256.

https://github.com/mongodb/mongo/blob/666003777e374d783ed1a450bcd4056d6ea00f95/src/mongo/db/s/resharding/resharding_server_parameters.idl#L37

Also change the default value of reshardingOplogBatchTaskCount to (reshardingRecipientServiceMaxThreadCount + 1) / 2. See the internalInsertMaxBatchSize server parameter as an example of this pattern.