By default one piece of C4 can be carried, but this can be increased to two by spending Certification Points in the C4 Cert tree. Engineers can carry up to a total of 4 charges with the fourth level of the Utility Pouch Certification.
It takes two charges of C4 to destroy a Lightning (or, if carefully placed, a heavy tank). Players may deploy as many C4 charges as they are certified to equip at one time. Attempting to deploy more will make the earliest one placed disappear. When two or more charges are deployed at the same time, the detonator will detonate all charges at the same time. C4 cannot be detonated after death, by other explosives or gunshots. It will simply disappear.
You may press right click to deploy the C4 and afterwards left click to detonate it.