Following are complete details of instances when a country has appeared in 15 or more Test matches in a 12-month period (since the inception of one-day cricket in January 1971), arranged in chronological order: 
Country         Tests     Period                                            

West Indies     15        November 28, 1975 - August 17, 1976 (6 Aust, 4    
                          Ind & 5 Eng)                                      

Australia       15        June 17, 1977 - May 3, 1978 (5 each vs Eng, Ind   
                          & WI)                                             

India           17        July 12, 1979 - February 19, 1980 (5 Eng, 6       
                          Aust, 6 Pak)                                      

Australia       16        September 11, 1979 - September 2, 1980 (6 Ind, 3  
                          WI, 3 Pak, 4 Eng)                                 

Australia       15        June 13, 1985 - March 17, 1986 (6 Eng, 6 NZ, 3    
                          Ind)                                              

England         16        February 21, 1986 - January 15, 1987 (5 WI, 3     
                          Ind, 3 NZ, 5 Aust)                                

Australia       16        August 17, 1992 - August 19, 1993 (3 SI, 5 WI, 3  
                          NZ, 5 Eng)                                        

West Indies     15        November 18, 1994 - August 28, 1995 (3 Ind, 2     
                          NZ, 4 Aust, 6 Eng)                                

India           15        June 6, 1996 - April 21, 1997 (3 Eng, 1 Aust, 6   
                          SA, 5 WI)

Total - 9 instances. Australia - 4, India - 2, West Indies - 2, England - 1

Note: Details in the brackets indicates number of matches played against different opponents by that particular country.

Story