EEG signals are small, ranging from 10s of microvolts to less than a millivolt. I'd advise against trying to amplify in one step with an instrumentation amplifier to a level sufficient for acquisition by ADC.
You need to account for electrode offset potentials. As a rule of thumb, I like to allow for about 150mV of offset. If you go much beyond a gain of 20, you're almost guaranteed to saturate your instrumentation amplifier.
The general approach should be modest gain with an instrumentation amp, followed by a bit of high-pass filtering to remove your offset, then some op-amp stages to bring the gain enough to record by ADC. This is, in fact, what's shown in step 2 of the instructable.