Use one Helm chart for ALL microservices?
Generally, a Helm chart would cover all your microservices - ideally, you should be able to deploy whole application in one chart. It may get a little messy if you include external 3rd party dependencies, where you would need to handle dependent charts, but I assume this is out of scope of your question.
More so, if your microservices are very similar you could write for loops, such as:
{{- range $index, $service := .Values.myservices}}
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp-{{ $service.name }}
# --- etc ---
{{- end }}

Now, where to put your chart - in most cases a preferred way is to have a separate repo for all Ops CD files, including Helm chart - so this is where Helm chart would live. This is not to be mixed with CI files, such as Dockerfile - those should live alongside microservice repositories themselves.