We don't know exactly how it is enforced, but the limit appears to be both "per taxpayer" and "per program." In other words, if I install Turbotax on my computer and try to prepare tax returns for 10 people with different SSNs, only the first 5 will be e-filable. But I can't get around that limit by buying a second copy of Turbotax and installing it on a second, separate computer.
Even if you could discover a trick to e-file more than 5 returns (and I can think of several things I could try, although I don't know if they would work), it would be a violation of IRS regulations, so it doesn't seem wise to push it.
