Term "market share" usually refers to sales and this is how I used it. iPhone market share is 10+%. There is no need to guesstimate this. Information is easily available. The number of current users may differ (in part because iPhone market share was higher not so long ago and those phones are still in use) but in a long run the percentage should be close.
The only “market share” numbers that matter are active users. Those are devices that will (in this case) provide data for traffic.
Since Google and Apple have provided these numbers we already know active users for Apple is way above 10%.