How does an art portfolio factor into admissions chances?

In the case of the portfolio, I think some colleges value the portfolio submission more than a personal interview because it either shows or discounts evidence of your creative process and results. Therefore, whomever sees the portfolio will evaluate you against your peers and other applicants and make a judgement call. It's not as bad as having to Dance, Sing, or Perform your instrument in a live audition for the school, but the scrutiny will be the same because it's supposed to be your very best work and an accurate representation of what you are capable today without further instruction.

My advice to you is to focus on curating your very best pieces in the mediums you best work in. If the school wants different examples in different mediums, that is a more difficult task because you may only draw or paint a certain way and have never used oil paints or acrylics.

If you are a very skilled artist then I think, it's good to back up your portfolio submission with the awards and honors you have received for your submitted pieces. For instance, you can write in the notes, Piece #1, Won 1st place for Watercolor at this arts festival, or Piece #10, selected as a the graphic use in the City's new DEI marketing campaign. Or if you have won a scholarship for your arts talents, document that as well.

The key thing is that whomever looks at your work has to come away thinking this person has both talent and potential to grow as an artist with further instruction.

I would say your arts portfolio if you are applying to become an arts major is going to be worth at least 1/3rd of your your overall application weight. If you are a truly gifted artist, colleges are not going to hold you to having a 4.0 UWGPA and 1500+SAT test scores just to get into the art school. The better the portfolio the better chances you will get admitted into the best art programs in the country.

And if you are a certifiable genius who just paints for fun but wants to apply to the Brown/RISD dual degree program, you will probably not get admitted to RISD because it's highly competitive. It think the admit rate is like 2% for that program.

Good luck.