In true REST, you should probably POST this in multiple separate calls. The reason is that each one will result in a new representation. How would you expect to get that back otherwise.
Each post should return the resultant resource location:
POST -> New Resource Location
POST -> New Resource Location
...
However, if you need a bulk, then create a bulk. Be dogmatic where possible, but if not, pragmatism gets the job done. If you get too hung up on dogmatism, then you never get anything done.
Here is a similar question
Here is one that suggests HTTP Pipelining to make this more efficient