I am staking 400k coins 24/7 since 7 days, my reward time sometimes goes to 4 hours and then changes back to 2 days...
I haven't received staking rewards yet

All the other coins I started staking with since 7 days got rewards. 1337, RDD, Blitz, Grc,...
Only espers is giving me nothing.
You have very few coins
now you're hurting my feelings

I could have had more than a million but I managed to buy the att...
What is the APR for this coin? How much would you recommend to buy?
I don't know what an "att" or "APR" is, but there is no minimum amount of coins to be able to stake. A fix to the current PoS system is probably coming next week or so.
This is a chance to buy a esp2 at such a price
I agree that ESP may be undervalued but I'm far from giving investment advice here

Buy as much as you like, or not.
att was a typo, meant ath.
APR: annual percentage rate, basically the yearly interest/dividen.