Before buying a lot of manga, I think a few questions to consider are:
- How much grammar do you know?
- How easily (quickly) do you pick up/understand new grammar you learn?
- Roughly how many vocabulary words do you know?
If you are on the lower end of the grammar knowledge spectrum, and you want to start reading manga, I recommend buying a copy of Japanese the Manga Way, and reading through it first. It lightly teaches a lot of the most common grammar you’ll encounter when reading manga. It uses panels from actual manga, with full breakdowns of the dialogue, to show the grammar in action.
From there, I’d recommend looking into getting a manga that has been read (or is being read) in the Absolute Beginner Book Club. That gives you a wealth of grammar and knowledge targeted to a specific manga volume in the form of weekly discussion threads. And if you have any questions not already asked, you ask in the appropriate thread, even if it’s been over a year since the book club completed reading the manga.
At that point, once you have had exposure to the most common grammar you’re likely to encounter, and you’ve worked your way through one whole manga volume, beginning to end, you’ll be in a good place to decide what pace you want to go from there.
As for vocabulary, the fewer words you know, the more you’ll have to look up, and the longer it will take to get through each volume of manga.
If Box of Manga auto-bills monthly (I assume so as they refer to their service as a monthly subscription), then you could be in danger of getting more manga (over time) than you’re going to be able to read any time soon. If you can afford it, and if you’ll keep learning Japanese, this isn’t the worst problem in the world.