To double the number of +1/+1 counters on a creature, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.

If enough -1/-1 counters are put on Big Mother Mouser at the same time to make its toughness 0 or less (or the damage marked on it to be lethal), the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many Robot tokens you get. For example, if there are three +1/+1 counters on Big Mother Mouser and it gets four -1/-1 counters, you'll get three Robot tokens. That's because Big Mother Mouser's last ability checks Big Mother Mouser's existence just before it leaves the battlefield, and it still has all those counters on it at that point.