If I understand you question right, client has to be a dimension

Since your fact may or may not have have captured orders from all clients, it is not possible to get a reliable value for count of all clients from the fact.

to find the number of clients with orders-> you have to query the fact.

to find the number of clients->you have to query the clients dimension.

to find the number of clients by joining the dimension and fact use a left join on client dimension.