This the updated website from the Thai Embassy in London. There’s at date no official specific restriction regarding entries by air, but there’s a general restriction that states just two entries on visa exemption by any means. This is not correct and there’s no official statement from MFA supporting this.
Formally it’s two entities by land in a calendar year, and there’s no formal restriction or limit on entries by air. But sooner or later you’ll get pulled aside and questioned, told to find a visa and/or get denied entry.
*************************************************