Your new beer is 24 plato and your volume is 12.5 liters. To assume that your slurry is healthy it will have 0.8 to 2 billion cells per millilitres. Let's err and assume 1 billion cells. For ale a standard recommended amount of cells is 0.75 million per millilitres per degree plato. The calculation is,

pitching rate x millilitre of wort x degree plato of wort =cells needed.

that's 750,000 X 12,500 X 24 = 225,000,000,000.

225 billion cells. So if we assumed you have 1 billion cells per millilitre in your slurry then you need 225 millilitres of slurry.