Add property to get number of users in an application

tests/models/test_application.py

@@ -0,0 +1,22 @@
+from atst.domain.environments import Environments
+from tests.factories import ApplicationFactory, UserFactory
+
+
+def test_application_num_users():
+    application = ApplicationFactory.create(
+        environments=[{"name": "dev"}, {"name": "staging"}, {"name": "prod"}]
+    )
+    assert application.num_users == 0
+
+    first_env = application.environments[0]
+    user1 = UserFactory()
+    Environments.add_member(first_env, user1, "developer")
+    assert application.num_users == 1
+
+    second_env = application.environments[-1]
+    Environments.add_member(second_env, user1, "developer")
+    assert application.num_users == 1
+
+    user2 = UserFactory()
+    Environments.add_member(second_env, user2, "developer")
+    assert application.num_users == 2