You're mistaken about some parishes not having a church. According to the 1860 Federal census, the State of Louisiana reported 372 churches representing 11 denominations (Catholic, Jewish, Lutheran &c). Some parishes (Calcasieu, St Bernard, St Charles and St John Baptiste) had only one church. All other had at least three, and most many more than that. Total seating capacity of all Louisiana churches - over 206 thousand.

Christ in the Camp contains more valuable material along these lines than I once thought.