Just had this happen, but got 7 planets, not 6.
I originally played with a 16 planet, then tried the bread basket start, which was a nice start. But with all of the techs that allow you to cultivate bad tiles, I figured a 3 planet start nets you the most tiles overall, so gave it a try. On my second 3 planet start, I didn't colonize the 2 bonus planets right away because you don't get ideology points for them. And eventually ended up with 7 sponsors on my home shipyard.