How many E1 channels can STM-1 frame transport?

For very simple explanation:
STM-1 contains 3x TUG3 (TUG – Tributary unit groups).
Each TUG3 contains 7x TUG2.
Each TUG contains 3x VC12 (Virtual Container)
VC12 – contains E1 as payload.
Then 3x7x3=63 E1.

For more info you can see SDH Multiplexing.