In most places, it is illegal to "carry" a rifle except out shooting at the range, hunting, or where ever you are shooting that is safe (woods, field, etc.). Meaning... you can't carry a loaded rifle in your car and an unloaded gun is not going to do you much good.
In you home, I wouldn't be all that bothered to depend on a Marlin M60 for my safety. Sure there are better calibers and so forth, but you have a M60 and can probably hit what you point it at. Velocitors are my choice. If they are not available, I would go with standard HV copper plated CCI 22LR or LR HP's.
Most have no idea what they would do in an actual self defense situation.... If you feel you have to shoot to defend yourself (you are in fear of your life), I would start pulling the trigger fast and not look to see what the impact is. If they are standing, you keep shooting. They go down, you probably have time to flee and call the police. You don't "finish them off". When the threat is eliminated, you stop. If you are afraid they still might do something... get out of there ASAP and call the police. No poking the victim with the barrel to see if they move...
This is said in jest of course, but people actually do stuff like this.
How many shots? I don't know. The gun holds 15 rounds.
I kept a loaded 22 in my apartment for years until I got something more powerful and even then I still kept that 22 loaded because I was confident I could hit what I am aiming at with it.
Eventually if you have the interest, you'll probably buy something else to substitute for the 22 rifle. Pump action shotguns work well indoors. Handguns work well. It is about personal preference and what you are comfortable with.