I am from Ukraine.When crossing the border, I did not get a stamp in my passport – do I have to get a certificate of the date of crossing the border from the Border Guard before going to get PESEL?

If your entry to Poland has not been registered by the Border Guard during the border control, you should go to any commune authority and submit an application for a PESEL number within 90 days from the date of entry. By submitting this application, you will also be able to apply for a registration of your stay. The commune authority should issue a confirmation of submitting the application.