Can men/boys leave sperm on the chair that they sit on and can the next girl who sits on it get pregnant?

To be successful sperm must remain healthy in a moist environment like where they are produced or the female genital tract where they are injected. They dry out & die when exposed to air for any length of time. If the sperm was fresh/moist & somehow placed where they could enter the female genital tract it might be a problem. If not, it wouldn't.

Agree with Dr. F. What you described is not conducive to pregnancy. Have a healthy diet, exercise 30 minutes/day, drink plenty of water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/. Get HPV and Covid vaccines. Wish you good health!