Hello,

there is no standard way for that, I guess.

I would do that in the following way (programatically):

1. Add a new int field, which is default is '1' and it is readonly

2. Add this as a column in tree view with sum = Total

Although, it would be shown as a separate column, it would do what you seek for

Kindly use the search_count method:

self.env['res.partner'].search_count([('customer', '=', True)])

I hope that answers your question.

By using Group by Customer  (from add customer group)