Pros and cons of doing PhD in a research group with a high number of post-docs?
There are too many variables for a general answer. How do the post-docs work? Do they work together? Is the lab environment friendly and helpful or cut-throat. Are they willing to help you? Are they new at this or beginners? All the combinations could balance out to something good or bad. 

But most important is how interested your advisor is in helping you and spending the necessary time for it. The rest is probably just noise. 

But a cooperative staff can be quite helpful if you are getting started. There is a lot of experience to draw on since the post-docs come from different institutions. 

But do what you can to keep everything friendly and helpful. Establish a working relationship with your advisor. Don't be shy. Ask for help when you need it. Give help when you can.