Say I have 50,000 posts in my current WordPress installation. May be it will grow more than 5,00,000 post’s in the future. Now I wanna Scale the database.

I wanna put ,
1-50,000 posts > database server 1
50,001- 1,00,000posts > database server 2
1,00,001- 1,50,000 > database server 3
N posts – N posts > database server N
And, when I will go to WordPress dashboard, I can see the total post count .. combined 3 database post … Say 1,50,000 post’s… Also, when an user will browse my site…. Different posts will fetch from different database…. (Only Read)….

Now, (write query)-
When I will write post… From WordPress admin, the post will get stored in another database server. Now, If I see … Write Database server reached 60,000 or 70,000 posts … I Will simply put this server in ‘read’ database cluster….
I will get another server for “write” , again 60,000 or 70,000 posts …simply put it ‘read’ database cluster….
Like this N….