As many as you have memory for when u plug in your iphone to your computer, open iTunes and on the left under "devices" your iphone should be there. Click that and look at the bottom of itunes and there will be a little bar that gets full when your memory is getting full. If it is white for most of the bar, you should be good for a while. Most apps aren't huge.
This really depends on the memory of the iPhone handset as there are different GB.s storage capacity on different handsets starting from 8Gb
There are 15,000 Applications!
That depends on the storage of your iPhone
The iPhone has thousands of free applications, including food diary applications. Fatseeker and GoMealsHD are two apps that are free and have received good reviews. There are many more using the search feature on the iPhone as well.
You can not send songs from iphone, that are stored in itunes
As many as you want. I currently have over 500 on mine.
Almost all of the applications on the iTunes store is compatible with the iPhone 4. Exceptions are applications which are for the iPad.
Yes.
Yes It Does, If You Download To Many Apps Or Run To Many Applications At Once It Might Glitch
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Two of the highest seeling iPhone applications include Text Lingo and Angry Birds. See 'related links' below.
No, not all applications are priced. Many useful apps are free.