Session length: If your session is only an hour or two, you probably won't have time to change more than once or twice. However, if you have a longer session, you may want to bring 2-3 outfits so you can have more variety in your photos.
Locations: If you're shooting in a variety of locations, you'll want to bring outfits that will work well in each setting. For example, if you're shooting indoors and outdoors, you'll want to bring something formal for the indoor shots and something more casual for the outdoor shots.
Personal style: Ultimately, the number of outfits you bring should reflect your personal style. If you're someone who likes to dress up, you may want to bring more formal outfits. If you're more casual, you may want to bring more relaxed outfits.

Here are a few outfit ideas for your senior session to get you started:

No matter what you choose to wear, make sure you feel confident and comfortable in your clothes. This will help you relax and enjoy your senior portrait session.

Choose clothes that fit well. Clothes that are too tight or too loose will not look flattering in photos.
Choose clothes that are in good condition. Holes, stains, and wrinkles & large patterns or logos will distract from your photos.
Choose clothes that are appropriate for the occasion. If you're not sure what to wear, ask your photographer for suggestions.
Have fun with it! Your senior portraits are important, so choose outfits that make you feel happy and confident.