What portion of everything did Abram give to Melchizedek?

Answer: One tenth

Abram gave Melchizedek a tenth of everything; Hebrews 7:4 calls it a tenth of the plunder.

“and blessed be God Most High, who has delivered your enemies into your hand.” Then Abram gave Melchizedek a tenth of everything. Consider how great Melchizedek was: Even the patriarch Abraham gave him a tenth of the plunder.”— Genesis 14:20; Hebrews 7:4 (BSB)