The answer is two generations.
In the alternation of generations, there is alteration between two generations - haploid gametophyte and diploid sporophyte. Gametophyte produces haploid gametes which fuse to form a diploid zygote. Therefore, gametophyte reproduces with gametes. The zygote grows into diploid sporophyte which produces haploid spores. Sporophyte reproduces with spores, and new gametophyte develops.