well, kakashi was able to create like 30 shadowclones (or more) when he was alrdy out of chakra.... remember the fight against zabuza? and the angry mob after that?

I don't think that the amount of clones has to do anything with it, but more what you do with your clones.
The advantage of this kind of training is, that every single clone uses a huge amount of chakra for training and not just him alone, remember when he tried to learn the rasengan, he was TOTALLY exhausted even though he trained by himself without using kage-bushin...now imagine him doing the same kind of training with 100 shadowclones.. there is now way he is NOT using the nine-tails