Of course, this depends on what you exactly mean.

Do you mean, all the machines that decides a **specific** language? e.g., 
$$ L = \{ \langle M \rangle \mid M \text{ decides the language } A\}$$

then, it depends on the language $A$. For instance, if $A=HP$, the halting problem, then $L$ is clearly decidable (i.e., it is empty).

But if you mean, **any** language, i.e., that $M$ is a decider,
 $$ L = \{ \langle M \rangle \mid M \text{ halts on all inputs } \}$$
then $L$ is not recognizable, see Yuval's answer.