The update file is a SQL script file. It will be too big to run as single script, so the process breaks it down into blocks. The bigger the block the faster it will process, but too big a block will cause the process to fail. On systems tests, somewhere between 20 and 50 seems about right, but it may vary depending on individual systems. If 50 works then you can try a larger number, but take a backup first. If 50 doesn’t work then try 1. This will take a long time to run, but if it works, it proves that the problem is the block size not the database or update file. Next time try with a higher number (say 20).