Are women more fertile or less fertile after they have a first baby?

No more, no less: In general, having a first baby does not, by itself, make a woman more or less fertile than she was before. However, it's important to remember that fertility decreases progressively as age increases; how much and how fast will depend on age at the time of delivery, how much time passes before trying to get pregnant again, and whether other fertility factors arise in the meantime.

Probably more: This is a very general answer, but seemingly fertility is better if one has had a child. One has, after all, demonstrated that they can be pregnant and that the "system" works. There are many exceptions, of course, but often once one has conceived they have more success with subsequent attempts.