10 is the minimum requirement. However, 10 messages is really low and they are unable to judge your character and maturity with that little information.
50 messages is a decent amount to get at least a bit of information out - depending on what the player has posted and commented.
Imagine your 50 messages were replying to games only thread, just for the sake of getting the counts up - those kind are normally ignored as they do not contribute much to the server/forums.
(Quality over Quantity)
What they look for in a player is firstly their maturity in handling given situations. How they deal with rejection and disagreements.
From there, they can judge what kind of character you are, if you're quick tampered, understanding, toxic and etc.
They are unable to reply every single staff application as they may not be able to read every single one of them. One player could have posted more than 1 applications which would bury the others. It is already hard to look out of potential staff members so please understand when they do not respond to it. They are certainly not ignoring you but they also do not want to give you false hope.
> Sees inbox from staff handling the application
> Gets excited
> Realized you failed.
> Gets depressed
I'm understand your frustration as I too have applied for helper (when I had about 10 messages) but had to go away for about 1-2 months. I never reapplied after the new batch so I used that time to to get others to know more about me first and hopefully like who I am and what I do.
So my tip for you if you do not get a response, do not give up. You may get a little envious and depressed but remember there is always a next time. Use that time frame to show others your potential and hopefully in the future, they may consider you.