I have been thinking of playing Tera but I want to know if it is free to play and not p2w? Also how is the player population?

Depends on how you look at it, it is technically P2W because you are able to buy instance reset scroll which allows you to run a dungeon over again, and buy cash shop items which consists of mounts, consumables and costumes which could be sold to obtain gold. It's also subscription base as well which allows you to gain 100% exp boost, reputation boost and gold boost, which are all purchasable to free users. In that context it's free to play but it's a lot of daily grinding at end game.

So far where I have looked most people say that it isn't pay to win? But what you mentioned I am not happy to hear..
Although as long as people can't spend money to buy the best gear than it should be ok.

it's completely free to play but paying for subscription gives you lots of perks. what people do is they buy EMP and they sell it for gold, and if you think of it this way, then it is pay to win since you are able to obtain upgrades for your gear at a much faster pace than you would normally without paying.

I see.. But so far this is the least pay to win, free game out there that I have found and looks to my liking.
Am I right?

Most definitely, least favorite pay to win game i've played. I've been playing it on and off for some odd years and tbh if you want to get great PVE gear you can't P2W for it, you'd have to join a guild that will help you with dungeons or you can PVP for PVP gear.