When this happened in RSA, the combined subscribers were less by abt 3 million. I think with the Kenyan culture of multiple SIMS and after Zain's onslaught, our total subscriber base could drop by 1-2 million.
We all remember the fine print in the SCOM IPO documents which indicated that its subscriber number (then 14 million) were actually SIM cards issued and not active subs. An easy way to work this out is to look at all the prefixes and assume 999,999 subscribers per prefix....start with 0720, 0721, 0722, 0723....0729, 0710, etc and multiply by 999,999 then we start to get the picture...
Ditto Zain - if they have reached 0737, there should be over 6 million subs...
I think a good part of these SIMS are those that came with handset offers...some because of the novelty of it (e.g. I have a Yu number that I have only used thrice - the novelty was being able to specify the number on the website).