Why am i missing some dungeons?

i recently bought the battle chest that includes burning crusade and the wrath of the linch king and lately ive been doing dungeons. right now im a level 40 and im wondering why do i only have like 6 dungeons which are the classics and do i unlock more later or are they just missing i know some disappear as you level up because you cant do them anymore. but still why are ther only 6 dungeons i looked it up and there are plenty more with the expansions i have