The amount of steem produced every day will stay the same. A fixed amount of steem is being distributed to posters/curators every day. When you get a posting/curating reward, you get a portion of this fixed amount (how big the slice of that pie will be is determined by votes). When people buy steem they buy it from other people, through a marketplace/exhange. The higher the demand for steem, the higher the price. When there are more steemit users, you will receive less steem as a reward. Because the amount distributed daily is still the same, but it now has to be shared among more people. The actual value of the rewards will stay the same, because although you get less steem, that steem will be worth more.

I hope I explained these basics clear enough. You should be able to answer your own questions after understanding this.