Come on buddy, you need to give us a lot more then just how many days and where you are landing if you want us to suggest anything, and without knowing what type of traveler you are, and where you will be going, it's next to impossible to give an accurate budget.
What's made you want to come to Thailand? What research have you done and from that research whats taken your interest?
What are your interests
I would suggest at least 3 full days in Bangkok (+possibly a side day trip to Ayutthaya)
At least 4 days Chiang Mai/Chiang Rai
and the rest Phuket with maybe a day or two visiting Islands