Some 313 DC-8s are still registered as "in use" I suppose that some of these might currently be for sale or owned for spare recovery but the right around 300 mark is a pretty good figure. Well over half of the DC-8s built are still in service, many more than civilian 707s. The majority of 707s are later build KC-135s and remain in military service.
The DC-8 is by far the most durable and longest lasting of the first generation 4 holers. I bet that if the VC-10 had been more commercially successful it too would have continued to serve long after 707s were gone. Of course there still are some 25 VC-10s in RAF service.