You'll have to replay a lot to get up to 99,999 NOs. In my first playthrough, despite spamming the NO! button most of the time, even during cutscenes, I only managed about 2000 total.

There's 8 unmissable story-related achievements that unlock as you complete each chapter of the game. The rest of the hidden achievements, in the order that they appear in-game, are:

Customize your character before starting the game.

In the prologue "Wizards of Nay" when you reach the janitor imp, win this achieve just like Luigi would -- by doing absolutely nothing.

Channel even more of your Luigi powers by doing absolutely nothing during 10 different "conversations" with your coworkers.

When the Coach asks you to subscribe, like and comment... do absolutely nothing.

Once you learn Laughing, use it against 10 different coworkers.

A coworker named Alex shows up immediately after the guy who insults your outfit (but was actually just projecting). Do nothing twice to begin playing their Pokemon clone prototype game.

Shortly after the Manager "fixes" your office with a "window" in Chapter 2, a coworker will challenge you to a staring contest. At this point in the guide, you can probably guess what (not) to do.

Shortly after the staring contest, after knocking over and climbing a pillar to the top floor, Graham will be the first coworker you encounter. Do nothing to accept the letter, then tell Zaim NO! when he tries to brush you off.

For the last question of the "debate" in Chapter 5, do nothing at all, and watch as poor Noah coughs up a full $50 to pay for the CEO's parking tickets. Oof.

In Chapter 6, in the brick hallway right after the coworker who wants a pen, you'll be approached by a third member of the Staring Club. It's unclear if you need to win any of the other staring contests first (i.e. Chapters 2 and 5), but at this point in the guide you should already be an absolute grand master of staring blankly at your coworkers.