What is the minimum number of blue tags I need to put on an image (so that it doesn't cause me problems in the future)?

five

Thank you!

Do keep in mind that the more tags you add, the better it is both for those who wanna search for that particular image, and for you, too, in case you wanna dig deep into your favourites collection.
Having a large amount of images/GIFs/animations, the majority of them tagged only with the obligatory 5 voices, is gonna make it hard for you, too.

The more the better, that's a general rule I try and follow. But the final decision's up to you, of course!

That's very true, and I can provide an example of it. I uploaded this image on August 22nd:

gelbooru.com/index.php?page=post&s=view&id=5496630

Then on the 23rd I discovered a sparsely tagged variant of it that had been posted on the 14th. I then added all the tags from my upload:

gelbooru.com/index.php?page=post&s=view&id=5477457

Despite being the same drawing with color variations my upload has a score of 28, while the one uploaded first only has 18. More people found the one I uploaded shortly after it was uploaded because of the extra tags, and it made a pretty big difference in the image's score. Once an image is a few pages down, it gets a lot less traffic.