Hello This isn't search function. It doesn't know number of occurences. It paint same search as selected. Nothing else. If you will keep selection and you will scroll over your document (e.g. with mouse), you will see occurences over whole document. But I don't know number of them. Highlighting is done only on visible part of page.
If you mean when you select text, PSpad should search over whole document, count occurences an do it every time when you change selection, it isn't possible. Can you imagine to do it in few MB large document? -- <https://forum.pspad.com/read.php?2,73695,73697> PSPad freeware editor https://www.pspad.com