Hello @ogonzalesdiaz ,
The Batch size field lets you select the number of inbound transition elements to be updated per database transaction. In a single database transaction the number mentioned in Batch size will try to update/insert to database.
The default (max) is 10,000 after which it will just iterate to the remainder and then process them as a separate batch call in the activity.
Changing the value can increase or decrease the performance of the database for some operations under certain circumstances.
There is no fixed number which can be recommended as best practice. it is dependent on how good the data model is built. It may work fine for 50k in some cases and it may degrade the DB performance in 20k.
Hope this information is helpful.