This is a huge market. Not every case is the same. These are $100m+ contracts being negotiated. People have been yearning for a second source if for anything but as a bargaining chip. Shifting from Intel to AMD is a joke compared to shifting from Intel to Power8/9, and IBM has been recording profits in their hardware division and expects a growth to double digit marketshare by 2020. Google uses Power8 for instance.
In the context of Zen.. AMD doesn't need much of the market to declare Zen a success in servers, 5-10% would be huge (they are less than 1% currently), and it's certainly achievable provided they have a good product. Or do we forget that at one point they had over 20% of the market share? I personally worked with racks of Sun Opteron servers back then, and they were great, no code changes needed, stuff just ran better and consumed less power than it did on competing Intel servers. We ran mission critical telco software on these.. with FCC fines if stuff broke, no issue.
If Zen can match perf/watt of Intel in datacenter it will sell. Period. Most businesses will continue to buy Intel as they always do, but that's not what I am talking about.