Helm pass values to the templates for multiple environment - kubernetes

Question

I am very newbie on Helm and struggling to configure deployment.yaml.Mychart tree structure looks like below. But how should I pass the values for dev and prod to the deployment.yaml?
For example if I would like to use different replicas for prod should I add another values such as below or deployment.yaml always keep as it is and use mutlipe values.yaml like as below.
replicas: {{ .Values.replicaCount .values.dev.replicacount }}
Or use only below tag is enough. Let's say if git branch equals to master then use below command
helm install . -f values.production.yaml
If git branch equal to development then use following command
helm install . -f values.dev.yaml
+-- charts
| \-- my-chart
| +-- Chart.yaml # Helm chart metadata
| +-- templates
| | \-- ...
| +-- values.yaml # default values
| +-- values.dev.yaml # development override values
| +-- values.prod.yaml # production override values

Answer 1

You should have a values.yaml file per environment.
That means that in your templates/deployment.yaml you'll have
replicas: {{ .Values.replicaCount }}
And then, for each environment you'll have a specific values.yaml. Like:
+-- values.yaml # default values
+-- values.dev.yaml # development override values
+-- values.prod.yaml # production override values

Answer 2

It really depends on the differences between your environments.
As being described in helm docs:
There are three potential sources of values:
A chart's values.yaml file
A values file supplied by helm install -f or helm upgrade -f on helm install or helm upgrade
The values passed to a --set or --set-string flag
At a high level, you might want consider the two approaches below:
If your environments have major differences between them then passing a different values.yaml file is a option and worth the extra maintenance.
On the other hand, if the differences are only with a few fields - consider using just one base values.yaml file with default values and override just the specific fields with the --set flag.
(*) In your specific case, you divided the DEV/PROD configuration into different files which is a good practice but there are cases where the difference can be only with one or two urls (and maybe some secrets that you'll want to pass as inline values anyway). So you can save yourself the extra maintenance.