Promise.all

api.get<ClientsResponse>('clients').then(response => {
      console.log(response.data.clients.length);
      if (response.data.clients.length == 0) {
        setNumberClients(0)
      }
      setNumberClients(response.data.clients.length)
    });

    api.get<ContactsResponse>('contacts').then(response => {
      console.log(response.data.contacts.length);
      if (response.data.contacts.length == 0) {
        setNumberClients(0)
      }
      setNumberClients(response.data.contacts.length)
    });