It's an Optional Rule in SR4A. It's an actual rule in place for Missions.
Edge Dice are not modifiers. theyre... Special. Edge tends to break the rules, so ignore them under most circumstances.
For everything else, it's 20 after modifiers (Positive and negative), or twice your natural attribute plus skill, as the rule says. Most of the time, that limits runners to 20 dice, since it's tough to get above a skill of 7 and a natural attribute of 12 or so. (Note that this is natural, not augmented as well).
Additionally, resistance tests (Body and Willpower) are exempt from this rule.
Now, that said... I encourage players NOT to cheese. Missions are not written with cheesemonkeys in mind. In general, if you're min-maxing your characters to the breaking point, you are likely to start breaking Missions if they're being run straight (And at conventions, they usually are). And frankly, I find that games aren't fun when I break them, and they even less fun when other players do it.
However, for a Missions game taht's largely a home game? Well, it's your call as GM. Hopefully you're modifying and adapting the game to your players particular... Cheesetastic tendencies
Bull