Perhaps someone with the Leadership: Apostles book can elaborate, but I believe from what I've read it was at least Jim and Dennis. To quote one letter:
McCotter has recently (1984?) sought to tighten control over GCI and the individual groups by giving the whole movement a "denominational" tag--Great Commission International--and promulgating new teaching concerning apostles and elders. His apostleship teaching is a reversal that has taken place over the past two years or so. Now McCotter has found it convenient to declare tht the Holy Spirit does still make some men apostles, and that he is the apostle of GCI. This legitimizes his dictating to the individual groups/churches around the country, as well, as to the individual members. Dennis Clark is also an apostle in GCI but not with the same standing as McCotter. McCotter claims to be an apostle like Paul; but Dennis claims to be an apostle of music (whatever that is).