AWS Lambda read from SQS without concurrency

Question

My requirements are as follows.

Every two hours, all the messages in a SQS are read, collected, and processed.
A file containing information from SQS messages is created as part of processing, and it is then uploaded to a sftp server.

I used an AWS Lambda to implement step 1 of the process. My Lambda contains a sqs trigger. I've set the batch timeframe to 2 hours and the batch size to 50. My presumption was that Lambda would be activated every two hours, that it would receive 50 messages all at once, and that I would create a file for every 50 records.

But I observed that my lambda function is triggered with varied number of messages(sometimes 50 sometimes 20, sometimes 5 etc) even though I have configured batch size as 50.
After reading some documentation I got to know(I am not sure) that there are 5 long polling connections which lambda spawns to read from SQS and this is causing this behaviour of lambda function being triggered with varied number of messages.

My question is

1. Is my assumption on 5 parallel connections being established correct? If yes, is there a way I can control it? I want this to happen in a single thread / connection
2. If 1 is not possible, what other alternative do I have here. I do not want to have one file created for every few records. I want one file to be generated every two hours with all the messages in sqs.