First you have to figure out the volume occupied by one paper clip you were only given basically the length. You need to know the diameter of the wire the the paper clip is made from.
If d is the diameter of the wire in inches and L is the total (unwound) length of the wire then the volume of the paper clip is Vc=πd2L/4 to convert this, which is in in3 to litres multiply by 1.639x10-2 now
you have the volume of one paperclip in l's. If you now divide 6.5 by this number you will have an estimate of how many clips might fit in the jar.The exact number will depend on the geometry of the jar and how closely you can pack the clips in it. For example
if the wire were 1/32" dia. and the length were 2.5" the approx number of paperclips the would fit is 207,000.
Hope this helps