Currently, this is set to 10,000 in https://github.com/GoogleCloudPlatform/spanner-migration-tool/blob/89802cf90407ca0785e426a0044d03da35f59e80/spanner/writer/batchwriter.go#L37. We should increase it to a higher limit as Spanner now supports 80,000 mutations per commit ([doc](https://cloud.google.com/spanner/quotas#limits-for)).
Currently, this is set to 10,000 in
spanner-migration-tool/spanner/writer/batchwriter.go
Line 37 in 89802cf