How much are these onion cell magnified?

It depends on the strength of the lense you are using and how high you set the magnification of the microscope.
At 40X magnification, the microscope enlarges a 2 mm part of the onion and at 100X the microscope enlarges 0.8 mm of the cell. At both 40X and 100X you can online see a zoomed out image of the cells.

At 400X, the microscope enlarges a 0.2 mm part of the cell. The individual cell is more clearly seen.
At 1000X magnification, the microscope enlarges a 0.08 mm part of the onion cell. At this level of magnification the nucleus of the onion cell can be seen.

This link shows different magnifications of the onion cells.