There is an official rule of 2 by land, and there is no official rule for air, but it is still close to the 2 by land. You had a total of 3 this year. You weren't given a 4th one.
I just re-read your post above. The times counted is the # of visa-exemptions you are given. When you extend it (an extension), it's not a new visa-exemption stamp. You just pay to make it longer.
You applied for a re-entry permit each time (I don't think it's even possible to can get one for a visa-exemption stamp)? Or a brand new visa-exemption stamp when entering?
You can't leave the country, and then extend the visa you had before you left.
First, the number of visa-exemption entries by land per year allowed is maximum 2 per calendar year (no official rule by air, but still).
Second, one huge mistake you did was trying to enter by the Cambodian Poipet border point. This is the worst land entries in all of Thailand. Many reports of people having issues, getting bribed, refused, and so on.
It's difficult to understand how you say you were in Thailand 5 months this year, doing 30 days and 30-day extensions, but you say you've entered Thailand 5 times so far.
Your trip in February is separate from the trip you are going to take. If you received a tourist visa (and haven't entered Thailand with it yet, in other words, haven't used it), it's still valid and you can enter Thailand during the 90 days of which the visa is valid (90-day countdown starts the day it is issued to you).
If you enter Thailand before it expires, you'll be able to extend it another 30 days at immigration for 1,900฿.
You have a Single-Entry tourist visa right now, but haven't used it? If you're received an SETV eVisa, it's valid for 90 days, and you can enter the country any time within those 90 days.
If you'll be getting an SETV before coming here, yes, you'll be able to extend the 60-day SETV for another 30 days at immigration for 1,900฿.
Same thing for the visa-exemption stamp, but if you enter in October, it'll be 45 days instead, and you can extend for another 30 days for 1,900฿.
It's a requirement everywhere. Not with police directly (that would be a bit extreme), but in the TM30 system. Simply ask your landlord to provide a TM30 for you. You print it out and then go to the back at immigration to do a "TM30 receipt" (which they/you staple in your passport and take photocopies of when need to).
You need to ask for a new TM30 and do a new "TM30 receipt" every time you stay at a hotel, since the hotel staff will do a TM30 during your stay at the hotel.
