I think the most structured way would be to have a dedicated table of these sales routes.
Now create a Cartesian relationship (x operator) from the SalesRoute table to your layout table, so you can display all records in a portal. This also allows you to use them as buttons.
If you add a summary field "Count of" to the Client table, and add a table occurrence of Client behind the SalesRoute table, you can display that summary field in the portal; it will show the count of related clients for each route. To recapitulate: you need
LayoutTable x SalesRoute = Customer
LayoutTable::anyField x SalesRoute::anyField, and
SalesRoute::id = Client_forDashboard::id_salesRoute (or SalesRoute::name = Client_forDashboard::territory)
Each portal row can now be used with Go to Related Record [ from: Client_forDashboard ]
Hope that makes sense.
this may be a rare occasion where a repeating field would be handy. My thought is that you could populate a number repeating field (x15) with client count (via script). Then, on your layout, you could use Merge fields like so:
<<Table::RepeatingNumber>> through <<Table::RepeatingNumber>>
(there are probably better ways to do this ... hopefully, you already ID your territories 1-15)
p.s. the repeating field would use global repeating storage