Not really sure if I understood you question. Hopefully this helps.
If you need to add multiple clients,
tb_clientes tb = new tb_clients();
entidade.tb_clientes.Add(tb);
tb = new tb_clients(); //create another
entidade.tb_clientes.Add(tb); //add the 2nd one
entidade.SaveChanges(); // you only need to call this once to save all changes.
You also mentioned you had a customer and want to add clientes to customer. You'll have to set the customer otherwise your clientes will remain orphaned.
tb.Customer = yourCustomer;
If you do the above, and assuming yourCustomer is already in the context, you will not need this line
entidade.tb_clientes.Add(tb);
Since EF will know what you intended to do and automatically add tb to the context for you.
You can also add clientes directly to the customer
tb_clientes tb = new tb_clientes();
yourCustomer.Clients.Add(tb);
tb = new tb_clients(); //create another
yourCustomer.Clients.Add(tb); //add the 2nd one
entidade.SaveChanges();
Edits in response to OP's comments
From your images, it looks like you are creating a many-many relationship between the tables.
Will your tb_clients_contatos table contain payload in the future? If not, the easiest thing would be to drop the contato_clientID column and make the remain 2 columns PK. If you update the EF at this point, it will create a many-many relationship automatically. You will not have to deal with this table since EF will maintain tb_clients_contatos for you transparently. Here's the new code
tb_clientes tb = new tb_clientes();
yourCustomer.Clients.Add(tb); //you're done at this point.
//A row in tb_clients_contatos will automatically be inserted when you SaveChanges.
tb.Customers.Add(yourCustomer); //this will work too.
If your relationship tables do need payloads, here's a good post on it. Create code first, many to many, with additional fields in association table