Edit. It was certainly possible to have 20 segments, but I thought I recalledl more being possible. The oldest rule sheet I can immediately find is 2006, which had the 20 limit. And that was flight segments.

The 16 segment thing (including surface segments) was imposed by the "enhancement" of e-ticketing. Once e-ticketing arrived there was an overlapping period where paper tickets could also be issued, but then they could only be 16 segments (so there was no advantage to paper ticketing).