@Paul ********
, they surely get at least one (there are still a bit of exception though)
What I mean is, some offices MIGHT have the policy of 1 same extension per 1 visa, others have 1 same extension per entry.
If you have a single entry visa, it wouldn't do anything with your extension application. But if you have a multiple entry visa, it MIGHT (or might not) affect your plan.
For example: immigration office A has 1 extension per 1 entry policy. Immigration office B has 1 extension per 1 visa policy. You enter Thailand using a Multiple (<- not single) entry visa. You are staying in office A area at the first, every time you enter Thailand, you apply for an extension with no problem. But at the final entry from your visa, you happened to stay in office B area. Then you try to apply for an extension for the last entry, but from office B's point of view, you already got an extension from that visa, office B wouldn't give you an extension. And it is not easy for everyone to move around inside Thailand to just apply for an exntentiosn.
So just check with the immigration office where you would stay for the last with your multiple entry visa, and make your plan.