Initially after Camp Jackson, a call goes out for the MSG, the militia that becomes the nucleus of Missouri Confederates. It's hard to definitely tell its strength, but would think 40,000 is a realistic number.....out of a population a little over 1,000,000 in 1860. Of that 40,000 likely 20-25,000 entered Confederate service from MSG. As things were chaotic, some changed minds and returned home, as well Union realized chaotic situation and that the call for MSG was legal, so offered amnesty if they would quit and return home. During this period there was little guerrilla activity.
However during period, jawhawker outlaws raided from Kansas, out of state troops often treated slaveowners as disloyal without reason. The frustration of Union being unable to swiftly deal with MSG lead to punitive policies against population. So by spring of 1862 a population that had been Union leaning, started to have doubts, guerrillas became widespread, as support for them.
Study of situation here is odd, as authors often contradict themselves. The war was essentially inheritly political, Republican vrs Democrat more then north or south.