Let's say we have the page. During rendering the page, we need to execute about 15 requests to API for getting some data.
How does this number of requests will affect on performance for desktop/mobile versions? Do I need to do any changes for reducing the number of requests? It will be great if you can send me the link with clarification related to this theme.
Optimization is this case really depends on the result of the API calls. Like what you are getting in response. Is the same static data each time or is it the same data with slight changes or is it extremely weird data which changes in real time?
There are many optimization techniques like whether to use Sync or Async, caching, batching, payload reduction. There could be many more but I know the few above. You can get a lot about these with a single Google query. It is up to you to decide which to use and where to use.
Various browsers have various limits for maximum connections per host name; you can find the exact numbers at http://www.browserscope.org/?category=network
Here is an interesting article about connection limitations from web performance expert Steve Souders http://www.stevesouders.com/blog/2008/03/20/roundup-on-parallel-connections/
12 requests to one domain/service is not much. latest versions of browsers supports around 6 simultaneous http 1.x connections per domain. So that means, your first 6 service calls (to a particular domain) needs to be done before initiating the next HTTP connection to that domain. (With HTTP2, this limitation will not be there though). So if your application is not intended to be high performing you are usually fine.
On the other hand, if every milli seconds counts, then it's better to have an edge service / GraphQL (my preference) aggregates all the services and send to the browser.
