The information you have isn't quite right.
The official documentation on this is at:
http://support.apple.com/kb/HT4515
What does that documentation mean? Well...
On AT&T 2G AND 3G (or "4G" as they insist on calling it), you can conference in up to 5 people. Technically, any GSM carrier can do this, even on 2G. This is because GSM was designed to mimic the functions of
ISDN lines, back when they were supposed to be THE thing that replaced regular
POTS and
dialup.
With GSM, you can also use features like call Add, Swap, and Merge. You can also take individual people (or two or three people) into a separate, private "sidebar" to discuss things separately, in the middle of a conference call, and you can even drop specific people off the call while keeping the conference going.
On Verizon, Sprint, and most other CDMA carriers, you can conference in only yourself and 2 other people. Doesn't matter if it's 2G or 3G. None of the other features (add, swap, merge, private, drop) are available, either. This is a throwback from a decision made by CDMA carriers to be as backward-compatible to analog cell networks and phone lines as possible. It's a decision that made sense at the time, but the only way to revisit that now is to push forward with voice over LTE.
But right now, no North American carrier does voice over LTE... YET. The iPhone 5 technically supports it, but the networks (at this time) do not.
It's unknown what the capabilities of VoLTE will be in terms of conference calling. Verizon/Sprint might choose to enhance conference calling. Or, AT&T might decide that 5-way calling isn't a needed feature. No one really knows yet. It's really up to the carriers, what features they offer and when.