In Appian, the maximum number of rows that you can retrieve in an expression rule depends on the database that you are using.
For example, if you are using a MySQL database, the maximum number of rows that you can retrieve in a single SELECT statement is determined by the value of the max_allowed_packet system variable. The default value for max_allowed_packet is 4MB, so if you are using the default value, you can retrieve up to 4MB of data in a single SELECT statement.
If you need to retrieve more data than the maximum number of rows allowed by your database, you can use pagination to retrieve the data in smaller chunks.
It’s also worth noting that it’s generally not a good idea to retrieve large amounts of data from a database in a single query. It can lead to performance issues and make it difficult to work with the data in your Appian applications. Instead, it’s usually better to retrieve only the data that you need for a specific task and use filters and sorting to narrow down the results.