IMO the easiest implementation would always break into an index and sitemap(s) as needed. 49,999 URLs? 1 index and 1 sitemap. 50K+ URLs? 1 index and sitemaps as needed.
This gives you the most consistent output and users who cross the 50K threshold over time will not need to update their sitemap in search tools to be compliant since pointing to the index will already deal with that.
Comments
IMO the easiest implementation would always break into an index and sitemap(s) as needed. 49,999 URLs? 1 index and 1 sitemap. 50K+ URLs? 1 index and sitemaps as needed. This gives you the most consistent output and users who cross the 50K threshold over time will not need to update their sitemap in search tools to be compliant since pointing to the index will already deal with that.