The maximum number of records that can be deleted via Inline Editing is 400.
This can be tested by doing the following:
I. Create a CSV File that contains 1000 Records. See sample column header below.
1 John Smith
2 John Smith
3 John Smith
4 John Smith
5 John Smith
II. Create a Custom Record Type wherein Include Name Field= T.
1. Navigate to Customization > Lists, Records, & Fields > Record Types > New.
2. Give it a name (e.g. Sample Custom Record type)
3. Include Name Field= T.
4. Click Save.
III. Perform a CSV Import for custom record type created in Step II.
1. Navigate to Setup > Import/Export > Import CSV Records.
2. Import Type= Custom Records.
3. Record Type= select custom record type created in Step II.
4. Click Select and browse to the file.
5. Data Handling= Add.
6. Click Next.
7. Accept the default mapping.
8. Click Next.
9. Click Save & Run.
IV. Perform a mass delete via Inline Editing.
1. Navigate to Customization > Lists, Records, & Fields > Record Types.
2. Click List for custom record type created in Step II.
3. Set Inline Text= T.
4. In column header Name, there is a pencil icon which indicates that the user can perform Inline Editing on this column.
5. Hold down the shift key + Click the first row from the Name column > scroll down using the scroll bar + Click the 400th row (which will display a gridbox) > Press delete on the user's keyboard.
If the user tries to delete 500 rows via Inline Editing, nothing happens.