You are going to have to use 60 different triggers to create 20 different bombard cannons, 20 camels, and 20 jannisaries. This will create 60 units simultaneously.
However, like dead_end said, you can use a single trigger to create an unlimited amount of units of a particular type. All you have to do is create a looping trigger, and select the type of unit to be created. Then use task object to move the newly created units away from the creation point. As long as a unit is on the creation point, no new unit will be created. Therefore they have to be moved away in order to continue production. Finally, use a trigger with "Own Objects," select the type of unit, and set it to 20, so that once you reach 20 of those particular units it disables the create trigger. You will need 3 different triggers (1 for bombard cannon, 1 for camel, and 1 for jannisary) to create the units, and 3 different triggers to disable them, for a total of 6 triggers. This will not create the 60 units simultaneously; it will create them one by one.
[This message has been edited by dane_g87 (edited 07-04-2008 @ 07:20 PM).]