Is it correct to receive a 30-day stamp on a single entry Non-B visa in Thailand, and can I extend my stay?

When arriving in Thailand on a single entry Non-B visa, it's typical to receive a 30-day stamp upon entry. In your case, although the visa is valid for three months, the officer's decision to issue a 30-day stamp is correct. To extend your stay beyond this initial 30 days, you need to visit an immigration office to apply for a visa extension, which could grant you an additional 30 days. Alternatively, you could also leave Thailand and re-enter to gain a new stamp, but this would require another valid entry. If your work permit is still being processed, it's advisable to ensure compliance with all immigration regulations during your stay.