I apologize for my inaccurate answer.
Once you have reference to the pager you should reach the PageSize Field and set the desired text by its PageSizeText property, see sample below:
var pager = (RadListView1.FindControl("RadDataPager1") as RadDataPager);
foreach (var field in pager.Fields)
if (field is RadDataPagerPageSizeField)
(field as RadDataPagerPageSizeField).PageSizeText = "custom Text";
Progress is here for your business, like always. Read more
about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic.
Our thoughts here at Progress are with those affected by the outbreak.