Go to "My Computer/Program Files/KONAMI/Silent Hill 2" Open it. Choose the "options.ini" (This is a notebook, that says the option settings of the game.) After opening the "options.ini", you will see a text, that says:

SET DX_CONFIG_WEAPON_CONTROL 0
SET DX_CONFIG_MAP 1
SET DX_CONFIG_WALK_RUN 0
SET DX_CONFIG_VIEW 0
SET DX_CONFIG_BLOOD 0
SET DX_CONFIG_BULLET 0
SET DX_CONFIG_CONTROL_STYLE 0
SET DX_CONFIG_VIBRATION 3
SET DX_CONFIG_SUBTITLES 1
SET DX_CONFIG_BGM_VOLUME 25
SET DX_CONFIG_SE_VOLUME 25

-Now, in the stage: "SET DX_CONFIG_BULLET 0",
change it with increasing the number zero (0)into a highter number e.g.: (250)
After change, it must be says:
"SET DX_CONFIG_BULLET 250".
Save the changes and close the text "options.ini".

Now play the game with new or saved game. Take handgun rounds or another weapon's shells (e.g. Riffle shells). Now the number will be to up to 999 in your ammo!

Note: if you want to changes the settings at the game, in "Options/Game Options",
you will see the stage:"Bullet Adjust x250". You can increase the number, but Is better to not even tease that, maybe that return to zero (o) automatically.