Salesforce batch apex limitations

1. Up to 5 batch jobs can be queued or active concurrently.
2. Up to 100 Holding batch jobs can be held in the Apex flex queue.
3. A maximum of 50 million records can be returned in the Database.QueryLocator object. If more than 50 million records are returned, the batch job is immediately terminated and marked as Failed.
4. If the start method of the batch class returns a QueryLocator, the optional scope parameter of Database.executeBatch can have a maximum value of 2,000. If set to a higher value, Salesforce chunks the records returned by the QueryLocator into smaller batches of up to 2,000 records. If the start method of the batch class returns an iterable, the scope parameter value has no upper limit. However, if you use a high number, you can run into other limits. The optimal scope size is a factor of 2000, for example, 100, 200, 400 and so on.
5. If no size is specified with the optional scope parameter of Database.executeBatch, Salesforce chunks the records returned by the start method into batches of 200 records. The system then passes each batch to the execute method. Apex governor limits are reset for each execution of execute.
6. The start, execute, and finish methods can implement up to 100 callouts each.
7. Only one batch Apex job's start method can run at a time in an org. Batch jobs that haven’t started yet remain in the queue until they're started. This limit doesn’t cause any batch job to fail and execute methods of batch Apex jobs still run in parallel if more than one job is running.