What is the maximum zoom possible in digital cameras?

That would depend on the camera you buy but macro lenses are pretty incredible sometimes.

Generally the most I have seen is 8-10x Optical, except for some professional SLR Digitals.

You need to specify as to which digital camera. A digital SLR or POINT & SHOOT.
The maximum zoom in point & shoot is 10x -12x but these come with an optical image stabiliser. But 10x or 12x does not mean anything unless you know where your starting point is i.e.. 10 times what. It usually means 10x times the lowest focal length of the camera (usually 36mm givving you up to 360mm focal length) But the if the lowest focal length is 28mm it will give you max focal lentgh of 280mm at 10x. Less magnification but extra wide angle.
As for digital SLR the lenses are interchangeable and depending on the focal length you get the zoom, The maximum available is 800mm which is 19x zoom if you consider the normal eye vision is equivalent to 50mm.