Why can I only see a limited amount of leads/customers?

This could be down to a filter that has been pre-populated. On the right hand side of the Pipeline page, there is a dropdown that will filter the results so that you are only able to see leads or customers that belong to you.

To see all results, simply select All Users.

Alternatively, it could be that your user has been set up to only have visibility over certain Teams. If this is the case and you need access to other teams, you will need to speak to one of your SuperAdmin users to get this changed.