How many times can I apply for a NON-O Visa based on marriage per year in Thailand?

I stay in Thailand for 6 months a year 3 months at a time, I therefore get two Non-Os a year from RTE London

well, it is an unusual way to achieve a 90 days stay permit - because many people can get the same out of a free-of-costs visa-exempt entry and a 1900 THB extension for 30 more days. I reckon it is the continuous 90-days stay that is attracting you (no need to visit Immigration) . . well, it is as Brandon quoted - you can get it as long as the embassy where you apply is willing to issue the visa consecutively within a short span of time. As there are more than 6 months in between, I reckon you will be fine

The limit is as many times as the embassy will sell you the visa.

That should be ok