Maximum size of Index?

#target framemaker

main ("Index");

function main (markerType) {

    var book, doc, count;
	
	book = app.ActiveBook;
	if (book.ObjectValid () === 1) {
	    count = processBook (book, count, markerType);
		alert (count + " " + markerType + " markers in book.", "www.frameexpert.com");
	}
	else {
		doc = app.ActiveDoc;
	    if (doc.ObjectValid () === 1) {
		    count = processDoc (doc, count, markerType);
		    alert (count + " " + markerType + " markers in document.", "www.frameexpert.com");
		}
	}
}

function processDoc (doc, count, markerType) {

	var marker;
	
	count = count ? count : 0;
	
	marker = doc.FirstMarkerInDoc;
	while (marker.ObjectValid () === 1) {
		if (marker.MarkerTypeId.Name === markerType) {
			count += 1;
		}
		marker = marker.NextMarkerInDoc;
	}
	
	return count;
}

function processBook (book, count, markerType) {

    var bookComp, doc, regex;
    
    // Regular expression for standard FrameMaker documents.
    regex = /\.fm$/i;
    
    // Loop through all of the components in the book.
    bookComp = book.FirstComponentInBook;
    while (bookComp.ObjectValid () === 1) {
        if (bookComp.ExcludeBookComponent === 0) {
            if (regex.test (bookComp.Name) === true) {
                // Get the document returned in a JavaScript object.
                doc = CP.getDocument (bookComp.Name, undefined, false);
                if ((doc) && (doc.ObjectValid () === 1)) {
                    book.StatusLine = "Processing " + File (bookComp.Name).displayName;
                    // Call the function to get the document's data.
					count = processDoc (doc, count, markerType);
                    // If the document was opened by the script, close it.
                    if (doc.openedByScript === true) {
                        doc.Close (true);
                    }
                }
            }
        }
        bookComp = bookComp.NextBookComponentInDFSOrder;
    }

    // Reset the book status line.
    book.StatusLine = "";
	
	return count;
}

There may be the limit of maximum number of document objects. For the 32-bit versions of FM this is 2^24. For a calculations an index entry consists of some (probable 5-10) document objects. But IMHO this limit will not be reached by your index: 120pages * 2 columns * 50 lines * 10 objects is only 120 000 < 16 million...

The internal context table is a memory construct that holds many types of FrameMaker document objects. The following table illustrates the different entries and their requirements.
Element: 2 entries; Variable: 2 entries; Cross-reference: 2 entries; Marker (any type): 1 entry; Table: 1 entry; Aanchored frame: 1 entry; Hidden conditional text: 1 entry per block. In FrameMaker 5.5, maximum number of entries is 2^24 (16 777 216). This may be extended in the 64-bit version (since FM-15). See https://www.daube.ch/docu/fmaker25.html