Is there a convenient function in objective-c / cocoa-touch to find a lowest number?

If you're using ints, use the MIN() macro:

MIN(25, 50); //Returns 25

If you're comparing two NSNumbers, then use the compare: method:

NSNumber *number, *secondNumber; //Assume 'number'=25, 'secondNumber'=50
NSComparisonResult result = [number compare:secondNumber];

return (result==NSOrderedDescending)?secondNumber:number; //Returns the 'number' NSNumber