I suggest you read through this thread. I think it will help you. If you elect to not isolate her, please pay attention to the part about marking the eggs and checking under her daily to remove any other eggs.
Isolate a Broody? Thread
https://www.backyardchickens.com/forum/viewtopic.php?id=213218
Now to your specific questions.
I do not give a hen any eggs to hatch unless she spends two consecutive nights on the nest instead of roosting in her favorite spot. Many will act broody but I do not consider then committed enough to give them eggs until they spend two consecutive nights on the nest.
I don't know how many hens or roosters you have. I'm going to give you another link to another thread. This one shows you how to look for the bull's eye to see if an egg is fertile. The bull's eye may be hiding on the bottom of the egg yolk, so you might need to turn it ever gently with a spoon to find it. Obviously you cannot hatch an egg that has been cracked, but if most of the ones you check are fertile, odds are most that you put under the hen are fertile.
Fertile Egg Photos
https://www.backyardchickens.com/forum/viewtopic.php?id=16008&p=6
You really need to collect all the eggs you want her to hatch and start them at the same time. You can do a search for "staggered hatch" to see some of the headaches not doing that causes. Some people do it and have techniques to manage it, but I certainly do not advise it for your first time hatching.
The egg can cool off and still be viable. The best temperature to store an egg for incubation is around 50 to 55 degrees fahrenheit. They certainly do not need to be kept warm. Freezing is bad and it is best if they don't get too cold or stay too warm, but you would be surprised how tough these eggs are. I'm sorry but I'll give one more link. I know it is a lot of reading, but you are asking a lot of questions. Good questions. This one is a lot about using an incubator instead of a broody, but it talks quite a bit about storing the eggs for incubation. That part is valid whether you use a broody or an incubator.
Texas A&M Incubation site
http://gallus.tamu.edu/library/extpublications/b6092.pdf
Please remember these are guidelines, not absolute laws of nature. Many of us violate some of these guidelines and still do OK. If you follow the guidelines you are not guaranteed a great hatch. If you violate some of them, you are not guaranteed a total absolute failure. They are intended to improve yor odds, not guarantee anything. Consider them a target to aim for, not something you have to hit the bull's eye each time.
When I collect eggs to incubate, my house is above the gudeline temperatures. I put them in my coolest room and turn them daily, actually using the turner from my incubator.
How many eggs to give her? She needs to be able to cover them all comfortably. Different chickens and eggs come in different sizes. Some bantams can only cover 4 full sized eggs. Some full sized hens can cover 18 full sized eggs. For me, I usually go with 12 full sized eggs under a full sized hen.