The best would be to limit the data directly in the query, answered by another user.
If, and only if, you for some reason can not change the query/data, then just break out of the loop
function bp_group_list_admins( $group = false ) {
global $groups_template;
if ( empty( $group ) )
$group =& $groups_template->group;
if ( !empty( $group->admins ) ) { ?>
<ul id="group-admins">
<?php
$i = 0;
foreach( (array) $group->admins as $admin ) { ?>
<li>
<a href="<?php echo bp_core_get_user_domain( $admin->user_id, $admin->user_nicename, $admin->user_login ) ?>"><?php echo bp_core_fetch_avatar( array( 'item_id' => $admin->user_id, 'email' => $admin->user_email, 'alt' => sprintf( __( 'Profile picture of %s', 'buddypress' ), bp_core_get_user_displayname( $admin->user_id ) ) ) ) ?></a>
</li>
<?php
$i++;
if ($i >= 2) { break; }
} ?>
</ul>
<?php } else { ?>
<span class="activity">No Admins</span>
<?php } ?>