Configure V8 DefaultWorker threads in yaml

We are using the wasmervice bootstrap extension in yaml as below

bootstrap_extensions:
- name: envoy.bootstrap.wasm
typed_config:
"@type": type.googleapis.com/envoy.extensions.wasm.v3.WasmService
singleton: true

When we have this in the yaml even after marking "singleton : true" we see 16 "V8 DefaultWorker" threads started on a 32 cpu box.
Is there a way to limit these in the yaml with some parameter ?
Or do I have to change code and rebuild envoy ?
--concurrency doesn't help in configuring these as that is only for worker threads.
Tried doing a google search for this issue but nothing turns up.