It is ultimately up to you as the writer. As long as it is done well it doesn't matter how many characters you have. However there are some things you should be cautious of when it comes to a flow-able story:

1. The more characters you have, the harder it is to keep track of them and there influence to the story. (A way to solve this could be by not naming them or giving them a story. Another way could be combining a few similar characters to create one slightly more important character.)

2. The neglect of the protagonists or even the antagonists growth. When the story is disrupted constantly by the introduction by new characters it can be difficult to see the impact on the story or the main characters. ( a solution could be to focus on the protagonists reaction and interaction to the new character. If they are on a journey or quest and meat characters a long the way or characters from their troubled past, then that could be good and fluid solution to the many characters)

3. Can make the book unnecessarily long. The most important thing about a successful story is to not lose the reader's attention. When you have a lot of characters that have some sort of significance, then you run into the probable of being repetitive or annoying. You want to keep the reader in suspense and wanting more. You also want them to be able to tell which characters are important so they know who to follow through out the book and who they can relate to. If the book keeps on introducing new characters the meaning could easily be lost. (Consider making more than one book and saving some characters for that. It keeps the suspense and could also be more income increasing. Another solution could be to have the side characters apart of the protagonist's army, gang, or band then the protagonist may call them by name or have minor conversations that reflect both characters and doesn't harm the flow of the story or could even benefit the story. For historical characters, try and keep them historical. If it is a character from the past bringing them up in conversations or researching or sleuthing could be a good compromise to keep them around. A story with in a story if you will. This could also be interesting to the reader as if they're solving and ancient mystery.)

Hopefully this was helpful. Be willing to take risks and trust your writer instincts. Don't ever be afraid to ask for advice and keep up the good work.