Honestly, quite a lot still. You might not face gamebreaking bugs as much as when playtesting a game with multiple variables to keep track of, or branching paths to take into account. But anytime you have several thousands or lines of script to go through, both out of game (just reading though the raw renpy script) or in game (because it can change the way the dialogue is perceived), even going through it two or three times is rarely enough to fix all the obvious mistakes. Then you have to take into account repetitions, figures of speech that you find weird reading out loud...