How traffic is allocated for variations in AB Testing [on hold]

What type of algorithm used for traffic allocation or visitor bucketting in AB testing?

Example :

I have two web pages A.html and B.html and need to show specific percentage of visitor A.html and others B.html say 20% of visitor should see A.html and other 80% will see B.html.

I don't know how many customers will come but traffic allocation should be evenly distributed when visit count increases. i.e., when 200 visitors visits my page 40 (i.e., 20%) visitors should see A.html, when 500 visitors visits my page 100 visitors should see A.html.