Mass update can be done for more than 100000 records at a time depending on some factors like one mass update can be done at a time, complexity in search criteria, peak hours, and connectivity. If we are processing with many records it will delay the process of the job beginning. Once the process is started we can not stop or cancel the process.
As a solution for that it would be better to break up mass updates into smaller batches and schedule them during off-peak hours.