How much oil should be put into my 1999 Montero sport?

Giving the engine size would have helped narrow the answer.

But here you go.

1999 MITSUBISHI MONTERO SPORT 2.4L 4-cyl Engine Code G

CAPACITIES

Engine, with Wix Filter 51356..........4.6 quarts

1999 MITSUBISHI MONTERO SPORT 3.0L 6-cyl Engine Code H 24-valve

CAPACITIES

Engine, with Wix Filter 57092..........4.8 quarts

1999 MITSUBISHI MONTERO SPORT 3.5L 6-cyl Engine Code R DOHC

CAPACITIES

Engine, with Wix Filter 57092..........4.9 quarts

Hope This Helps