I read there was 7 seasons planned in total, but the writers strike messed it up.
This is what I read is happening as season 6 will be the final one:
"On May 7, 2007, ABC Entertainment President Stephen McPherson announced that Lost will end during the 2009–2010 season with a "highly anticipated and shocking finale."[63] "We felt that this was the only way to give Lost a proper creative conclusion," McPherson said.[63] Beginning with the 2007–2008 television season, the final 48 episodes would have been aired as three seasons with 16 episodes each, with Lost concluding in its sixth season. Due to the writers' strike, the fourth season featured 14 episodes, and Seasons 5 and 6 will have 17 episodes each.[11]
Executive producers Lindelof and Cuse stated that they "always envisioned Lost as a show with a beginning, middle, and end," and that by announcing when the show would end that viewers would "have the security of knowing that the story will play out as we've intended."[63] Lindelof and Cuse stated that securing the 2010 series-end date "was immensely liberating" and helped the series rediscover its focus.[64] Lindelof noted, "We're no longer stalling."[64] Matthew Fox stated that he is the only cast member who knows the ending of the show.[65]"