I have created a page in SharePoint and that page should be visible to only few employees (around 500).
What is the best way to achieve this without adding them manually to the permissions list?
You have to do this with a group, either in AD or in SharePoint. Maintaining it otherwise will be too much pain.
Your problem is getting all the users into a format that SharePoint's going to like. If you have clean firstname/lastname pairs or usernames, you could just paste it as a semicolon-delimited list, then come back in the morning to see if the peoplepicker has crashed.
I'd recommend building a loop in Powershell to add each user to the list.
If you don't have a clean list of firstname/lastname pairs or usernames then your problem isn't a SharePoint one.
You could use an Active Directory group and add that to the SharePoint Members/Users/Owners group or give that AD group permissions directly.
From your post (500 being considered "a few"), it seems that you are talking about a big company, possibly with multiple SharePoint farms.
I'd say that the best way is to start by understanding the logic behind your permission management.
If it's just a one time thing, maybe adding them manually is the right choice, even if it's not very exciting.
But most probably these people have been selected because they are members of specific groups (executives, experts), and you'll have similar requests in the future. In this case it would make sense to have AD groups that can be reused across farms or in other applications.