The Digivolution levels are as follows
Fresh - In-Training - Rookie - Champion - Ultimate - Mega
Ultimate is the sixth level and is the second you're not guaranteed. To get an Mega level Digimon you need your Ultimate level Digimon to get a certain amount of key points (stats needed to Digivolve) and if you do, they will Digivolve into the Mega level Digimon with the most acquired key points. If you fail to get enough key points for any of the Mega level Digimon, your Digimon will die and be reborn at the Fresh level. Whilst you may not be able to get a Mega in your first life span, you will eventually reach the stage where your reborn Fresh level Digimon already have the stats to become the Mega level meaning you just have to wait until they get there. If you're going to beat the story, you are guaranteed this.