C.G.
When I introduced my daughter to cereal at that same age, my ped's recommendation was to feed it to her until she lost interest in the feeding. Sometimes, she would eat just a couple tablespoons. Other times, she would eat like 6! It just depended on her appetite, mood, etc. We would always start with a small serving, 2 or 3 tbsps, and let her eat as much of it as she wanted. If she scarfed it down and clearly wanted more, we'd make another batch and continue with the process. Conversely, if she was done, she was done and we stopped with the feeding.
The key at this point in time is not for her to necessarily get any nutrition out of the solids (she's still getting the bulk of that from breastmilk or formula), it's more for practice. She needs to learn how to eat and this is the training ground. So don't worry about how much or how little of the solids she is eating. Just let her be your guide.
Have fun and enjoy...this is a great time in baby's development!