Sequencing center returns data in several files
Usually, a single sequencing library is created from each sample. Assuming you are using barcoded indexes to identify multiple samples, the index is attached to each library, and the libraries are pooled to make a library pool.  A library pool can be frozen, and is very stable.  

A pool can be sequenced multiple times by pipetting a portion of the pool and supplying it to the sequencing process.  Each time, a sample of the molecules in the pool will be sequenced.

Generally, it is true that sequencing runs are very repeatable, and that you will not suffer any batch effects.

If you want to check for batch effects you would need to analyse all of your sequence files separately, and check this first, or in parallel, to your main analysis.

One thing you might want to check is whether the same library pool was sequenced multiple times, or whether separate pools or libraries were made for each sequencing run.  In the second case, there may be more likely to be batch effects.