Create a Record Rule
Settings → Technical → Security → Record Rules
Create a new rule like this:
Name: Customers: Restrict to Own
Object: res.partner
Domain Filter: [('|'), ('user_id','=',user.id), ('create_uid','=',user.id)]
Groups: Assign it to your vendor group or custom group (e.g., Sales / User: Own Documents Only)
Suppose you have 100 clients:
User 1 has 50 → they’ll only see their 50 (others are hidden).
User 2 has 40 → only their 40.
User 3 has 10 → only their 10.
When they go to Sales → Customers or CRM → Customers, the view will automatically filter.