How many times do I need to apply for a visa from Laos to Thailand and back to Laos?

At least two visas are required, and most countries only have 1 or more. I suggest you apply for a multiple-entry visa.

The use of one-time visa is consistent with the record of only one entry. After the first entry, the Lao visa will be invalid. If you return to Laos after leaving the country, there is no valid Laos visa on your passport. The Lao Immigration Bureau will refuse you entry and you will leave Thailand. This dilemma is very troublesome, and in most cases it can only be solved under the coordination of the embassy.