I have more than 5 subdomains on a project, say developer, www, staff and some more, what is the best practice in creating sitemaps? ... Since the subdomains are several parts of the main application, what are the pros and cons of each, and what should I consider while choosing the one over the other? (one sitemap per subdomain or one for the whole site)
Since sub-domains attached to the main domain can represent different websites to the world, you're best to setup Sitemaps for each sub-domain for easiest accessibility.
Additionally, I believe its against the rules of Sitemaps to include an external link in a Sitemap belonging to a website, and a URL that contains a different sub-domain counts as an external URL.
As for best practice, if your website has a huge number of links, you'll want to make a master Sitemap file that references smaller Sitemap files where each of them references the individual links. This is because each Sitemap file has a maximum file size limit (of I think 50MB) and also the maximum number of URLs per Sitemap are also limited. I think that number is 50,000.
It is a bonus if you can compress the Sitemaps in gzip format. That way, when you submit them to gzip compliant search engines such as Google, then the Sitemap will be processed faster and the bandwidth consumption on the server will be lower.
Here are links related to sitemaps that may help you more: