Number of CO2 in pyrolysis
While it would be possible to decarboxylate almost any carboxylic acid with appropriate conditions, simple aliphatic acids are not readily decarboxylated. Carbonyl group in beta position facilitate decarboxylation due to formation of a cyclic transition state:
Now, your example has five carboxyl group, and each has a carbonyl in beta position (carboxyl also counts as a carbonyl compound). Let's look at the picture:

First we can remove the three top carboxyls marked with arrows (A). There is a carbonyl in the ring that will aid in the decarboxylation, so we will arrive to a compound at B) (I'm not saying that the reaction will proceed that way, it is just an explanation). As I said before, one carboxyl group can facilitate the removal of the other carboxyl. But once we remove one group, we get compound C). That compound does not have any groups that would help to get rid of the last carboxyl. So there is your answer.