Number of metal ions characterized by flame test among the following is ________. \( \mathrm{Sr^{2+}} \), \( \mathrm{Ba^{2+}} \), \( \mathrm{Ca^{2+}} \), \( \mathrm{Cu^{2+}} \), \( \mathrm{Zn^{2+}} \), \( \mathrm{Co^{2+}} \), \( \mathrm{Fe^{2+}} \)
