One only - however, pre-flattening there could be considerable numbers of layers making up the pictures content
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There are many resources and websites that offer information about Photoshop layers. Some of these websites that offer information are Adobe, Photoshop Essentials and Photoshop Cafe.
Open the Layers window.
Photoshop has ties with other Adobe software for media editing, animation, and authoring. The .PSD (Photoshop Document), Photoshop's native format, stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings. This is in contrast to many other file formats (e.g. .EPS or .GIF) that restrict content to provide streamlined, predictable functionality.
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This depends on your question....as it is not clear enough. Photoshop documents apply to Adobe Photoshop. Photoshop uses many different file formats but the most common file is called a "PSD", which stands for Photoshop Document. There is also PSB which is an extended larger version of PSD files and it stands for Photoshop Big.
.psd Photoshop has ties with other Adobe software for media editing, animation, and authoring. The .PSD (Photoshop Document), Photoshop's native format, stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings. This is in contrast to many other file formats (e.g. .EPS or .GIF) that restrict content to provide streamlined, predictable functionality.
I opened a new project in Photoshop Elements 6 and began adding layers. I'm at over 360 and it hasn't stopped yet :D My guess is, it will add as many layers as your computer can handle without crashing.
Every image contains at list one layer but you can have as many as you want. Layers are separate images which can contain transparent areas or different levels of transparency. People use layers to create compositions in Photoshop because every layer can be edited separately without affecting other layers or composition. Another advantage is because you can save layered image (PSD, PSB, TIFF ) and edit layers at any time you want. Photoshop offers several type of layers like Type layer so you can change typed text only for example. When it comes to save image you can flatten image or to save flattened version of image on disc and leave composition as it is with all layers for further editing at later time. In other words saved layered image serves as template which can be edited even after 10 or 20 years.
In many different ways, first is to place pictures in same document. To place picture above another you can from Bridge: choose 2 pictures (hold down Ctrl while clicking second picture) then go to Tools > Photoshop > Load Files into Photoshop Layers or from Photoshop: File > Scripts > Load Files into Stack..
The files created in Adobe Photoshop have a .psdextension. This extension primarily works only in photoshop, although other graphic viewing and editing softwares may offer reading compatibility for this format.
Extensions are small programs which can run inside Photoshop. Example is Mini Bridge, Kuler in Photoshop CS5 but there are and many 3rd party extensions. Sometimes people refers to file format as extension or file extension like .csh extension for example to save Custom Shapes.