Can salespeople be restricted to only their own customers?

This can easily be solved using ownership-based security roles. The difficult question is whether you should.

The arguments in favor

Competition between salespeople, sensitive customer relationships, or regulatory requirements in certain industries. In some organizations, this is a genuine need.

The cost

Two salespeople may end up contacting the same customer without knowing about each other. The person covering for someone who is off sick has no visibility. And no one can respond to the customer when their regular salesperson is unavailable. That is a real operating cost.

The middle-ground solution most people end up with

Everyone can view the information, but only the owner can make changes. This prevents duplicate work and allows team members to help each other while keeping responsibilities clear. This model works well for the vast majority of businesses.