There are many form limits that are imposed by PHP. It is not possible for use to mention every one of them in our tutorials. we can help you to identify them when you do encounter them though.
You miss the point. It's not that there is a limit to how many records the tutorial is designed to work with, but that there _may_ be a limit imposed by the server.
One server may have settings in place that cause the limit to be 20 records, where another server may have limits to cause the limit to be 1000 records. It is a server configuration issue.
It also depends on how many column per record you are updating. for example in your case, you are updating the Price and Qty columns, so the form will need to contain and ID hidden element, and a element for the price and qty columns, that 3 elements per record. with 342 records to be updated, you need 1026 element, plus a submit button:
342 * 3 = 1026 + 1 = 1027
so, if you server uses the default 1000 form elements, you should be able to set the update page to update 332 records:
332 * 3 = 996 + 1 = 997
But again, that depends on the server configuration, and the number of columns being updated
see this page for details on post_max_size and max_input_vars settings
For updating text records, the 8M (8 Megabyte) default setting for the post_max_size should be fine