BuddyPress.org

Hi,
You need to override this js function

 bp_legacy_theme_hide_comments(

There are many ways to do it. You can either put the following code in your theme’s js file

function bp_legacy_theme_hide_comments() {
	var comments_divs = jq('div.activity-comments'),
		parent_li, comment_lis, comment_count;
	
		var max_show = 2;
	
	if ( !comments_divs.length ) {
		return false;
	}

	comments_divs.each( function() {
		if ( jq(this).children('ul').children('li').length < max_show ) {
			return;
		}

		comments_div = jq(this);
		parent_li = comments_div.parents('#activity-stream > li');
		comment_lis = jq(this).children('ul').children('li');
		comment_count = ' ';

		if ( jq('#' + parent_li.attr('id') + ' a.acomment-reply span').length ) {
			comment_count = jq('#' + parent_li.attr('id') + ' a.acomment-reply span').html();
		}

		comment_lis.each( function(i) {
			/* Show the latest 5 root comments */
			if ( i < comment_lis.length - max_show ) {
				jq(this).addClass('hidden');
				jq(this).toggle();

				if ( !i ) {
					jq(this).before( '<li class="show-all"><a href="#' + parent_li.attr('id') + '/show-all/" title="' + BP_DTheme.show_all_comments + '">' + BP_DTheme.show_x_comments.replace( '%d', comment_count ) + '</a></li>' );
				}
			}
		});

	});
}

or you can put the following php code in your bp-custom.php/functions.php

function buddydev_override_legacy_theme_hide_comments_js() {
	?>
<script type="text/javascript">
function bp_legacy_theme_hide_comments() {
	var comments_divs = jq('div.activity-comments'),
		parent_li, comment_lis, comment_count;
	
		var max_show = 2;
	
	if ( !comments_divs.length ) {
		return false;
	}

	comments_divs.each( function() {
		if ( jq(this).children('ul').children('li').length < max_show ) {
			return;
		}

		comments_div = jq(this);
		parent_li = comments_div.parents('#activity-stream > li');
		comment_lis = jq(this).children('ul').children('li');
		comment_count = ' ';

		if ( jq('#' + parent_li.attr('id') + ' a.acomment-reply span').length ) {
			comment_count = jq('#' + parent_li.attr('id') + ' a.acomment-reply span').html();
		}

		comment_lis.each( function(i) {
			/* Show the latest 5 root comments */
			if ( i < comment_lis.length - max_show ) {
				jq(this).addClass('hidden');
				jq(this).toggle();

				if ( !i ) {
					jq(this).before( '<li class="show-all"><a href="#' + parent_li.attr('id') + '/show-all/" title="' + BP_DTheme.show_all_comments + '">' + BP_DTheme.show_x_comments.replace( '%d', comment_count ) + '</a></li>' );
				}
			}
		});

	});
}

</script>

<?php
	
}
add_action( 'bp_after_activity_loop', 'buddydev_override_legacy_theme_hide_comments_js' );

Both will work. you can change the max_show to any value. Hope that helps.