There are five rotors of three maybe chosen and inserted into any position. This give us 60 ways of using our rotors. At the M4 Model there are 336 ways of using eight rotors.
Each rotor can have 26 starting positions. This makes an additional 26*26*26 = 17576 ways of setting our chosen rotors.
Totally this amounts to us 1,054,560 combinations.
If we add the plugboard settings we get 
for six sockets (12
Letters).
If 1000 operators with captured machines tested four keys a minute 24 hours a day, it would take them 900 million years to try them all!
But there were weak points. Enigma is a simple swapping machine of an advanced type.