I am wondering if multiple records on htaccess is useful for 301 redirects.

The issue I see is the fact that old site files have 'html' extension and therefore are named named in a different way so an easy global redirect will not work, it needs to be one rule per file title.

This sites receives 1000 visits daily so have to take care not to penalize internet search engine.

RewriteEngine On
RewriteBase /
RewriteRule ^file1\.html$ http://www.domain.com/file1.php [R=301,NC,L]
RewriteRule ^file2\.html$ http://www.domain.com/file2.php [R=301,NC,L]
RewriteRule ^file3\.html$ http://www.domain.com/file3.php [R=301,NC,L]
RewriteRule ^file4\.html$ http://www.domain.com/file4.php [R=301,NC,L]

A php header rewrite won't act as that old files are html type.

I guess you could utilize some regex to lessen the amount of different RewriteRules you're using, as individuals are searching exactly the same way.

Inside your situation, only using that one may be OK :

RewriteRule ^(file1|file2|file3|file4)\.html$ http://www.metaboforte.com/$1.php [R=301,NC,L]

By doing this, you specify exactly what you would like to rewrite only have 1 RewriteRule.

Or, a little more generic :

RewriteRule ^file([0-9]*)\.html$ http://www.metaboforte.com/file$1.php [R=301,NC,L]

Which enables you to definitely define that you would like to rewrite every fileXYZ.html, with Abc several. (When I used '*', no number whatsoever would be taken into consideration with that rewrite rule if you would like a minumum of one number, you need to use '+')

You might make a move much more generic -- unsure you would like that, but something similar to this may do :

RewriteRule ^(.*?)\.html$ http://www.metaboforte.com/$1.php [R=301,NC,L]

Here, you're redirecting exactly what finish with .html

Why don't you just...

RewriteRule ^file([0-9]+)\.html$ http://www.metaboforte.com/file$1.php [R=301,NC,L]

Or, if you wish to rewrite everything around the old site...

RewriteRule ^(.*?)\.html$ http://www.metaboforte.com/$1.php [R=301,NC,L]

Maybe you can test the RewriteMap Directive