How many single entry tourist visas can I obtain in a year while in Thailand?

It's not about how many you can get in a year as you asked, but rather that you appear to be using them back-to-back for non-stop stays in the country, which is not the intention of the visa. It is always possible you will be questioned or denied because they feel you are using it to stay in the country for months at a time. From what you said, you will have stayed in the country for 6 months on 2 back-to-back SETVs with extensions on each, so you are trying to get a 3rd one allowing you to stay for a total of 8 or 9 months continuously.

Nobody knows, and one persons experience has no bearing on what yours will be. Might get 2, might get 3....You won't know until you are denied the next one.

Official there is no limit. Thai Consulates in the area have their own limits on how many back to back SETV they will give you. Most of the times you get first a warning stamp next to the visa indication that this is the last on at that consulate.