Not sure I understand that. Why would it be different? Did it originally only get 15 for S1 or something?
I assume it would be done 1 year per season like B5, so if Jeremiah gets picked up for seasons of 20 eps it will be 20.
If TPTB then decide it should be 25, JMS would presumably write some extra filler shows, or if they decide it should be 15 he will just drop some and shift important "arc" point to other eps.
When season is picked up I am sure JMS will have a rough idea of what important things have to happen through the season, and will write the scripts (or have others write the scripts) to suit the number of shows available. So even if each season were to have a different number of episodes, each season will still be 1 year.