How many students are typical for an advisor?
It depends on many factors:


Does the professor have any intense other duties like teaching, committee work, own research, being an editor, writing grants, etc. or can they focus on supervising?
Does the professor delegate supervisory work, e.g., do PhD students have postdocs or similar as a first person to go to?
Is there a healthy communication and collaboration within the group? Is everybody roughly informed what everybody else’s areas of expertise are so they can seek help (e.g., from the resident programming expert) directly without going through the supervisor? Is this encouraged by the supervisor?
How much supervision does a typical student in your field require?
How efficient is the professor in supervising?


Due to these factors, the group size you mention can work very well, but may also be a total disaster. As it’s impossible to judge these factors as an outsider, the best way is to talk to actual supervisees and look at the graduation statistics of the group (if available). If you are actually working in a professor’s group, you should be able to assess this better than anybody else.