The base chance of a critical hit is roughly 6.64% in Gen 2, 6.25% in Gen 3-6, and roughly 4.17% in Gen 7 onward.
In each game, +1 critical chance increases this number to 12.5%
In Gen 2-5, +2 is 25%, +3 is 33.3% (actually 33.2% in Gen II), and +4 is 50%, providing the maximum critical hit chance you can attain in these games.
In Gen 6 onward, +2 stages grants a 50% chance of a critical hit, and anything beyond that guarantees one.
This chart on Bulbapedia makes this information more digestible. There's a chart below it detailing what things can affect critical hit stages.
Because Super Luck and a Razor Claw will both grant +1 stage of critical hit chance, that combination will result in a 25% chance of critical hits in Gen 2-5, and a 50% chance of critical hits in Gen 6 onwards.
