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

The flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic colour the salt turns the flame of a Bunsen burner. The heat of the flame excites the electrons of the metal ions, causing them to emit visible light.

$$\begin{gathered} {\mathrm{Sr}}^{2+}-\mathrm{Crimson} \mathrm{red} \mathrm{colour} \\ {\mathrm{Cu}}^{2+}-\mathrm{Blue} \mathrm{green} \mathrm{colour} \\ {\mathrm{Ca}}^{2+}-\mathrm{Brick} \mathrm{red} \mathrm{colour} \\ {\mathrm{Ba}}^{2+}- \mathrm{Pale} \mathrm{green} \mathrm{colour} \end{gathered}$$

Hence, the answer is $4$.