There are some variables here that you need to decide...
For an "Art Print" ... Something that can be looked at carefully and up close you will want 200 ppi or better for a big print.
For a display print ... something that is intended to been seen from a normal viewing distance you can get away with less (120 ppi is probably fine).
For advertising (really long distance) you can even go smaller.
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PPI = Pixels per inch. For the final output. DPI can have variable meanings.
So it depends on the final result you want. If you can I would make up a test print. Pick up some pics from the net or here and stitch them together in various resolutions. Have them printed and them put them at the viewing distance you expect the picture to be placed. See what is acceptable to you.
http://www.ltlimagery.com/essentialscanningservice.htm
Has some reasonable charts but they are for 35mm, if you are shooting another size you will need to scale accordingly.
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FYI (Disclaimer) I am not endorsing the above scanning/printing site I just like their chart. I have never used them.