I started Jacko at 4 months just with rice cereal. I just read the directions on the packet which were 2 tbs!!! He ate the whole lot!!! It wasn't until afterwards I realised, perhaps a teaspoon was a gentler start for him :). Seriously, he'll eat until there's no more food, so I'm not sure how much you're meant to give them!
We just had purees (making sure there was pear or pumpkin in it so he didn't get too backed up) until now. He's still getting some mashes of whatever we're having at 6 months, but we're playing with food as well, sandwiches, salad, fruit and veggies, cheese, pasta... he's loving it and stops when he's had enough if he's doing it himself.