How many registers does DE10-standard board have? Anyone help me?

The DE10 uses a 5CSXFC6D6F31C6N device.

The Cyclone V Device Overview document (https://www.altera.com/en_us/pdfs/literature/hb/cyclone-v/cv_51001.pdf) gives you a part number decoder (page 14) which lists available resources for all devices in the series.