No. They are provisioned permanently once added and are used to handle query workloads. They cannot be automatically provisioned only at the time of cube processing.
You can manually scale down the number of replicas after peak periods or automate scaling using Azure Autoscale based on query demand if you need to reduce costs.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola