This thread might have some still good links to numbers batches you can do (and some you should just set a panda for and do the as they come in)
https://www.mturkcrowd.com/threads/pandas-to-keep-you-working-cheap-numbers-hits.155/
(I hope that link works).
I believe Zoltars require 5K hits (anyone? They're not up now for me to check), but once you get there do those like crazy.
A9s I believe don't have a hit requirement (anyone?). Those are good ones to get numbers up.
Best thing to do is check Turkopticon to do batches for requesters that don't have a reputation for rejections. Right now you don't want any as that can tank your approval rating and prevent you from doing lots of good hits.
Are you using scripts to find hits, like Mturk Suite? If not, those are your very best bet. If you've already answered all of these questions before my bad...I wasn't on forum much this week due to being on vacation and hosting Easter.