Without the context of the mission, this question is very difficult to answer usefully.
Is there a group called grp_1?
How is this script being called?
Are there more than 6 units in the group to begin with?
On the face of it there's nothing wrong with the syntax of your code, however it could be written in a much more elegant way, i.e.
hint "Alive checking script ON"
#loop
~1
? ({alive _x} count units grp_1) == 0 : hintsilent "They are all dead" ; exit
hintsilent format["%1 soldiers left", {Alive _x} count units grp_1]
goto "loop"