You can count the number of table rows by placing a number field in a table and summing up the number using the SUM function in the Calculated field.​​​​​​

Place a "Number" field in a table, then click Settings from the gear-shaped Field settings icon at the top right of the field.​​​​

Enter a string of your choice for the field name and the field code, and specify '1' as the default value.​​​​​​
In this example, enter "Count" for the filed name and the field code.​​​​

Place a Calculated field outside of the table, then click Settings from the gear-shaped Field settings icon at the top right of the field.

Enter a string of your choice for the field name, specify "SUM(Count)" as the formula, and save the settings.​​​​

Now, the number of table rows is calculated.