What does 'Number of Items to Process For Each Queue Item' mean when using the API Importer extension?
API CSV Import extension uses Job Scheduler to import the records in Civi. The job scheduler breaks down the whole csv into small batches(also known as queue item) to import to avoid timeout issue, memory excess and various reasons.
'Number of Items' is used to define how many records you want to process in single single queue. If you specify 2000 than there will be 25 queue item lined up in job scheduler to process 50k records with 2k in each queue. The 'Number of Items' you specify should respect the max time out and memory size of the server or it will always result in error. I prefer to set to 500 per batch for better results.
HTH
Pradeep