Remember that quality is more important than quantity!
I think it can be a bit easier to go overboard with toys for infants and toddlers, due to the volume you may receive from adult friends during baby showers and early birthday parties that may be more geared towards your family and friends than the baby's friends.
Some toys naturally become favorites (a preferred stuffed animal, for example). Others can be picked specifically for their educational value. You want to make sure that whatever toys you have can keep the child's attention. However, each child has a different attention span, so that can be hard to judge. It can also be helpful to have something that is familiar (and therefore reassuring), such as the stereotypical "security blanket".
The most important thing to realize is that you don't have to buy toys, ever. There are always ways to entertain a child, regardless of the age, without going out and purchasing some fancy toy. A big cardboard box is likely to provide as much entertainment as a $30 electronic gadget (much as I love me some electronic gadgets!), and will stimulate more use of creativity and imagination.
I suggest that as your child gets older, you focus on picking toys for their meta value. Crayons, legos, building blocks etc. are fun not for what they are, but what you can make out of them. Particularly if you combine them with other objects (example).
As for how many toys is "too much", I think the real issue you are asking about is more a function of how your child perceives the toys, rather than how many are there. If your child has 200 toys, but doesn't have a huge attachment to them, shares well, and doesn't throw a tantrum when she sees one she doesn't own, then I'd say its not too many (although where and how you store them may disagree!).
Unfortunately, I've seen the opposite happen, where parents were really careful not to overwhelm their child with tons of toys, and the child wound up coveting any toys they saw, and getting really upset when told they couldn't take them all home.
Remember that the toys are marketed towards the children as much as the adults, and most manufacturers know that getting the kid to do the sales pitch for them can be the most effective method of selling their product. Reducing exposure to commercials aimed at children's toys may help.
For the record, so far one of my son's favorite toys is a remote control for a DVD player we don't use anymore.