Number of metal ions characterized by flame test among the following is _________.
$$Sr^{2+}$$, $$Ba^{2+}$$, $$Ca^{2+}$$, $$Cu^{2+}$$, $$Zn^{2+}$$, $$Co^{2+}$$, $$Fe^{2+}$$

Metal ions characterized by flame test:

$$Sr^{2+}$$: crimson red — YES

$$Ba^{2+}$$: apple green — YES

$$Ca^{2+}$$: brick red — YES

$$Cu^{2+}$$: blue-green — YES

$$Zn^{2+}$$: no characteristic flame color — NO

$$Co^{2+}$$: no characteristic flame color — NO

$$Fe^{2+}$$: no characteristic flame color — NO

Total = 4.

Therefore, the answer is $$\boxed{4}$$.