I'm new to the hobby . I was considering buying an airbrush and compressor but then I discovered that Humbrol do a range of spray cans. I model mainly Luftwaffe and wondererd if anyone else has managed to undertake the hobby using just spray cans of 7/8 colours for the main camoflage work or am I asking too much from a fairly limited range of colours.

if the colors are close enough for your tastes, there is no technical reason not to use rattle cans. I've never used Humbrol, but I like the Tamiya spray paints for certain tasks.
the cost of the paint in rattle cans is certainly much higher than bulk paint, but then there is no clean up needed after each shoot, which is a big advantage.
that much less exposure to acetone, which is all to the good.

Spray cans are good for doing large surfaces. Their worst limitation is when you want to spray small places like landing gear bays or cockpits, and when you're doing complicated camouflage schemes like Luftwaffe mottles or wave patterns. You also can't vary the pressure, which means that sometimes you get too much paint on your model.

Once you've done a few models you'll discover these limitations, and how to work around them for the most part. If you intend to get serious about the hobby, I recommend that you get the airbrush, compressor and spray booth. The sooner you get practising, the sooner you'll get good at it