What export resolution (pixels per inch?)

Ian-or other mod, this question gets asked so regularly a FAQ would be a good idea.

You will not lose pixels because of what number you put in the box. Pixels per inch is meaningless for digital photography, because digital photos do not have inches!

If you really don't want to lose pixels, make sure you don't check the box that allows you to change the size of the export (I'm at work, but I think the box says "File Size")

The ppi is irrelevant unless you are printing, in which case set the require ppi for your printer in the print module.

The only exception would be if you are up sizing the files from their native resolution in LR using cms or inches in the export box. Avoid this and work in pixels and you don't need to concern yourself in anyway with pixels per inch. Just export at the size in pixels you require for the end usage you are putting the file to, or don't resize at all if you want the file to remain at its original dimensions.

The ppi is only a line of metadata, it has no effect on image quality and is only used by a printer to make a conversion from ppi to dpi (dots per inch) when making a print.