score:5
England had won its big battles against France in the Hundred Years' War (Crecy, Poitiers, Agincourt etc.), typically outnumbered 2 to 1 or even 3 to 1.
Likewise, Portugal had a lower population than Spain and was "always" outnumbered by Spain. This was particularly true in this battle because some of the Portuguese had defected to the Spanish side.
In those situations, you don't try to match the French or Spanish armies. Instead, you muster what men are available, try to make use of the terrain and better tactics, and hope that it is "enough" to win. In this case (and many others), it was.
Upvote:4
This article notes that most of the Portuguese nobility actually sided with King Juan I of Castile. (Juan had married the daughter of King Fernao of Portugal; when Fernao died without a son, Juan claimed the throne of Portugal on the basis of his marriage to Fernao's daughter.) So it seems likely that King Joao of Portugal -- whose base of support was primarily Lisbon -- was really limited in terms of how many Portuguese soldiers and knights he had on his side, since most of the nobility and their soldiers were either sitting it out or siding with the Castilian invaders.