I presume a pesel number is a sort of national identification number? Which is unique for you, and is also stated on your ID?

I usually travel by filling out my national identification number in the Interrail pass instead of the number of the ID or passport, since I know that number by heart.

I didn’t encounter a single problem with it, ever. So I wouldn’t bother to change it. It only get’s checked very rarely. It’s a number that proves your identity as much as your passport number does.

Pesel number is something which everyone in Poland has, but I should write ID number from my ID card. I'm afraid of getting some kind of punishment. But ID number and Pesel are in my ID card.

 Don’t worry. It’s an unique number, unique to you and mentioned on your ID-card. That’s as much proof as the ID-card number itself.

This almost never, ever get’s checked btw, that number. Only on very rare occasions, with a very thourough inspector. Just avoid mistakes in the number you put. Which number; either passport-, ID card number, or national identification number doesn’t really matter IMO. The information on the card differs a lot from country to country even.

All of them are unique numbers, proving your ID. You’re absolutely not the only one mixing this up.

You should be able to change the number if you cancel the activation of your pass and then activate again.

In addition, it's strongly recommended not to activate your pass (or a travel day) in advance and to wait until shortly before boarding your first train. You never know what will happen and since you can only cancel activation until 23:59 CET before validity starts, you won't be able anymore to cancel activation on your planned start day if something unexpected occurs.