Yes you can stack chicken breasts in the instant pot or arrange the chicken breast anyway you like in the instant pot.
You can cook chicken breasts and other chicken or meats in the instant pot either frozen or thawed.
Cooking frozen chicken in an instant pot takes around 22 minutes.
You put the frozen chicken in the instant pot and allow it to cook for 12 minutes and then you leave the chicken in the instant pot for another 10 minutes during natural release.
After the natural release has released the pressure which takes around 10 minutes then the chicken should be fully cooked and ready to eat.
The chicken will cook faster in the instant pot if you allow the chicken to thaw for awhile before putting it in the instant pot because when chicken or other meat is frozen it takes a bit longer for the instant pot to reach temperature.
But even when the chicken is frozen it should take only 12 minutes at most for the instant pot to reach the cooking temperature and cook the chicken.