I think G+ has more than 20M members.
5 minutes ago i've done some maths using socialstatistics.com's data.
I've compared the followers that some members had on 2011/07/12 to the followers that the same members have today.
2011-07-12:
- Mark Zuckerberg followers: 134,328
- Larry Page followers: 73,319
- Sergey Brin followers: 56,015
- Vic Gundotra followers: 38,302
- Mashable News followers: 27,390
Total followers: 329,354
Members on 2011/07/12: about 10 Millions
2011-07-20:
- Mark Zuckerberg followers: 272,925
- Larry Page followers: 150,064
- Sergey Brin followers: 102,794
- Vic Gundotra followers: 64,462
- Mashable News followers: 75,874
Total followers: 666,119
Members on 2011/07/20: about 20 Millions