The value of X is calculated only once, as Dragonhawk's last ability resolves.

You pay all costs and follow all timing rules for cards played this way. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.

The delayed triggered ability created by Dragonhawk's last ability counts only cards that were exiled by the ability that created it. It doesn't count cards exiled by previous instances of Dragonhawk's last ability or by the abilities of other Dragonhawks you may have controlled previously.