so it says you can run 2 imports at same time.
and 10 can be queued of up to 10mb each.
You can increase that size temporarily with support.
Should I only make 2 concurrent API calls with 10MB file each and once complete make the next two call .
It really depends.
If you have a clearly fixed number of imports, like you've already sliced one day's single file into 10MB slices and that's under 10 files, then I would upload them all at once. Again that's assuming you will not be uploading again for 24 hours.
But if you don't know how many files will come in and at what pace, you don't gain much by uploading in advance. Instead, poll for completion every minute and then upload when there's a slot available. 10MB files are so fast to upload that pre-queueing them has little advantage (if the limit were 100MB+ it would be another story).