If "plenty" is less than a gallon and a half, sure. And a toilet tank is not designed to hold potable water -- it could have lead, and will often have rust.
Probably best not use the water in the basin for anything other than handwashing WITH SOAP.
You can re-use "gray water" for washing -- yourself, clothes, etc. In an emergency, being springtime fresh is not a top priority. Then you can dump it back into the toilet tank you've just emptied and get another flush or two.
If you suspect an emergency is in the offing, fill the tub. That's 42 gallons of water, probably not the best available for drinking, but you can also use that for washing and refilling the toilet tank. Think about it -- you'll need to take care of biological necessities long before you run out of clean (or at least "clean enough") clothes.
I have a little water filter I bought for a trip to Thailand. That and my bathtub could keep me going for a week or two if I keep using the toilet, a month if I dig a privy out back. The usual guideline for drinking purposes is one gallon per person per day.
Ten gallons of water -- two standard-size buckets -- is a pretty handy thing to keep on hand. You don't need to expect a major emergency. A 24-hour power outage or a water main break, even if they don't cut off the supply, could easily compromise the safety of tap water.
There is also plenty of fresh water in your hot water heater. Just turn on the drainage fawcett and collect the water into a bucket.
And TURN IT OFF. You don't want your water heater turning into a boiler on you.
> If “plenty” is less than a gallon and a half, sure.
In an emergency, a gallon and a half will last a fairly long time. Worst-case scenario for Civil Defense emergencies is benchmarked at three days.
> And a toilet tank is not designed to hold potable water — it could have lead, and will often have rust.
As could your tap water if your pipes are old enough. No, you should not drink water from your toilet reservoir when it isn’t an emergency. Equally, if it is an emergency it is perfectly OK to drink it rather than suffer thirst or dehydration. Any greeblies in it (usually none) probably won’t harm you.
Fortunately there are a very limited number of things that can go wrong that would compromise the water supply. Water flows thanks to gravity, and the entire process end-to-end is remarkably lo-tech. Even if the power is out chances are excellent that the water will continue to flow no worries. Sewerage, too: toilets are one piece of household technology that are unlikely to fail in an emergency.
The fresh water supply is also one of the services what would be exceedingly difficult for terrorists to damage in any meaningful way. Not by contamination, not by disruption.