Hello @l_enzo33,
No worries! The serial number for the Spider V 20 is usually on a sticker inside the box or on the back panel label. If you can't find it, try connecting the unit to Line 6 Monkey via USB and see if the ESN (Electronic Serial Number) is displayed. If you still can't locate it, contacting Line 6 support might be your best bet.
Best Regards,
James Keen