You don't collect multipliers on tiles that have been played by you or your opponent in previous turns. They may ONLY be played once.
When you play all seven tiles (that is called a "bingo" BTW) you get a 35 pt bonus, plus whatever multipliers are under your tlles.
The absolute minimum a bingo could be worth is 41 pts and you have to work hard to do that playing 5 1pt tiles, 2 blanks, and connecting to a 1 pt tile sitting on a DL in the 3rd row/column in from the edge.
The maximum really depends on your imagination: