How many days of visa will I receive if I cross back from Laos into Thailand on a 30-day visa exempt?

4

Your visa exempt and you'll get another 30 days visa exemption when you cross the border

You will only get a visa if you apply for one at the embassy. But you can enter Thailand with no visa ( exempt) for 30 days and extend a further 30.

30 days in Laos and then another 30 days when you come back into Thailand

You're not on any visa if you have 30 days. You're on visa exempt which means no visa at all.

You can either go to your local immigration office and apply for a 30 day extension one time per entry.

Or you can cross the border and return and you'll receive a new 30 day stamp if you don't have a visa.

If you haven't requested an extension yet, you should do that first. You're only allowes to enter Thailand at a land border without a visa 2 times per calendar year. So if you do it now you'll only be allowed one more time this year. Might as well get your extension first and save your entries until needed.