Board Thread:Manga/@comment-5774380-20160808233126/@comment-5774380-20160809040528

Neetaku wrote:

Didn't chapter 82 make a big deal about them being out of Thunder Spears? That's why Isayama had to magic Hange back to life so she could save them with hers.

They still can produce more. Zeke and Reiner will not return until the next arc; at least that was what Zeke seemed to suggest when he said he would come for Eren "another day".

Fire Eater wrote:

So there was no way of splitting the serum between Erwin and Armin? Someone please clarify.

There are several theories about this, but Isayama avoided the issue by letting Zeke and Reiner escape, so even if the serum could be split, the Corps still could only save one person because they had only one Shifter in the menu.