No apostrophe should be used. A 'thank you' is a colloquial compound noun. To put an 's' on it is to signify the plural. The apostrophe is only used to express possession e.g. Bob's radio; or missing letter/s such as the 'no' omitted in 'can't' which in its full form is 'cannot'. Note 'its' does not take a possessive apostrophe. When it has an apostrophe it is a contraction of 'it is'.
No. A 'thank you' is a colloquial compound noun, like lounge room or room heater, and the 's' in 'thank yous' signifies the plural, so no apostrophe is used. Apostrophes are used to express possession e.g. Bob's car; or to indicate some letters have been omitted in a contra