Breastfeeding to formula?

Hi everyone!

Baby is 18 days old. He had formula the entire time at the hospital but eventually I started expressing breastmilk through pumping and now straight breastfeeding.

I do want to incorporate formula for the days I am not around and for when there isn’t any frozen EBM left.

I was thinking of breastfeeding and topping off with 10mL once a day but I started reading and some say to replace one feed with formula.

So I wanted to get everyone’s opinion/advice?

Thank you in advance!