How long can a packet of sliced ham be used once opened? When buying ham slices from supermarket, I get like 10 slices in them. I want to use it for a week. 
But the packing says when once opened it should be used within 2 days. So it seems like the 8 slices just always goes into bin. Is it safe for me to keep it from Monday to Friday? 
And is there a way to store them?
When you buy the sliced meat, take out what you'll use in two days and freeze the rest. If you put wax paper in between portions that you would eat in one serving, or use separate baggies, you only need to pull out what you need. The package might recommend against freezing (for best quality, not safety), but a week or two in the freezer will cause no harm at all.

Personally, I find the 2 day limit a bit extreme, a week should be fine (see luncheon meat on the accepted answer here: How long can I store a food in the pantry, refrigerator, or freezer?). But if you would rather not risk it, the above should work well for you.