Does the "passport need to valid for 6 month" apply for extensions? And if so, does the counting start(months left) from when you apply or receive the extension? 

You can apply for extension with any amount of months remaining on pp.

 

However the extension will only be granted till expiry date of pp. 

 

The extension starts from expiry of current permission of stay. 

 

Pretty crazy (imo)  to have pp expiring in  much less than 12 months time. 

The Thai law is that your passport must be valid for the duration of your intended stay. Thus, if your passport has a validity of less than the normal length of a visa or extension, the expiry date will match that of your passport. For a 30-day extension of a tourist entry, this is rarely an issue. For a one-year extension based on retirement, for instance, you should ensure that your passport is valid for at least a year before applying for the extension. Otherwise, you will not receive the full extension that would normally be granted.

16 hours ago, DrJack54 said:

You can apply for extension with any amount of months remaining on pp.

 

However the extension will only be granted till expiry date of pp. 

 

13 hours ago, BritTim said:

The Thai law is that your passport must be valid for the duration of your intended stay. Thus, if your passport has a validity of less than the normal length of a visa or extension, the expiry date will match that of your passport. For a 30-day extension of a tourist entry, this is rarely an issue. For a one-year extension based on retirement, for instance, you should ensure that your passport is valid for at least a year before applying for the extension. Otherwise, you will not receive the full extension that would normally be granted.

Both correct, you cannot be granted any permission to stay that exceeds the validity of your passport. Simple logic, simple rule.