Hi @Dovydas! Welcome to the KPN forum. This depends wholly on the status of the order. 

Whem you first receive your simcard you need to activate it. After this, the simcard will start working on the agreed upon date on which the order is complete. 

When ordering a new mobile number this is usually the day you activate the simcard. When you bring a mobile number from another provider this is usually a month after you sent the text to agree to port the number. 

All of this can be followed through mijnbestelling.kpn.com. 

Would you let me know here if this has answered your question? Or if you have any other questions?