You have posted this in the wrong section deary, it belongs in the livebearers section
I will help you none the less, Guppies are one of the many water bunies by my books, they will breed until there will be no more room in the tank to move...
To tell if a guppy is prego, you look towards to anal finnage, there you will either see a piece of dangling waste (j/k), or a black spot, the black spot is fry, now depending on the size of the gravid spot (the black area) will tell you whether she is having babies or not, you can also tell through the size of the female, if she looks like she is about to burst, you will be looking at a healthy batch of baby fry... I have heard people getting between 2 to 36 fry, it varies really, but thats about the range you will find it, it also depends upon the size of the guppy and how old she is...
She should give birth in a couple of month though, depending when she got prego, you will just have to watch her closely since you bought her preggy...
You will know when she is close to birthing by unsual behavior and such thing as these, you dont want to put her in a breeding trap/net too early though, because if she gets stressed out, she will abort and their goes your baby fry, and also could injure your guppy...
(Me on a personal note allow livebearer to birth in the main tank, making sure their are suitable hiding places for fry and then remove the fry from the main tank after the female finishes birthing)
Then you put them into a seperate rasiing tank if you have one, if not a breeding trap/net will do, and feed them liquidfied food that you can buy, or super crushed, fine flakes, feed them about four times aday in small amounts to maximise their growth rate
I hope this is of help to you, and good luck
