As temporadas 1 e 2 deveriam ter 24 episódios (a quantidade "normal" de episódios para um programa em rede), mas para a 1ª temporada o final se tornou um episódio duplo.
Em relação à 3ª temporada: eles planejaram 22 episódios, mas o final se tornou um episódio duplo. A Wikipedia observa o notável cronograma de transmissão :
In response to fan complaints about scheduling in the previous seasons, ABC decided to air the episodes without reruns, albeit in two separate blocks. In the United States, the first block consisted of six episodes aired on Wednesdays at 9:00 pm and after a twelve week break, the season continued with the remaining 16 episodes at 10:00 pm.
Em relação à 4ª temporada, Wikipedia diz :
Os 17 da quinta temporada são devidos aos dois episódios "desaparecidos" da 4ª temporada sendo redistribuídos para a 5ª temporada e a 6ª temporada.The season was originally planned to contain 16 episodes; eight were filmed before the start of the 2007–2008 Writers Guild of America strike. Following the strike's resolution, it was announced that only five more episodes would be produced to complete the season; however, the season finale's script was so long that network executives approved the production of a 14th episode as part of a three-hour season finale split over two nights.
Com relação à 6ª temporada, novamente citando a Wikipédia :
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 2007–2008 Writers Guild of America strike, the fourth season featured 14 episodes, and season 5 had 17 episodes. Season six was planned to have 17 episodes, too. However, on June 29, 2009 it was announced that the final season would feature an additional hour, making the number of episodes 18.