No stable form can be produced by a single glider (on an infinite or toroidal board), thus a(1) = 0.
Block, boat, beehive, loaf, eater, and pond are the six still-life forms that can be made from 2 gliders.
Tub, Ship, Barge, Long boat, Mango, Long barge, Very long boat, Hat, Half bakery, Paperclip, and Bi-pond can be made from 3 gliders.
Most of the terms in this sequence are conjectural and will likely increase over time. - Nathaniel Johnston, Dec 03 2019