This really is just a math problem more than a bitcoin question, but it’s simple enough so here is your answer:

If i have $1000 worth of bitcoin with the current price of bitcoin at $7000

($1000)/($7000/BTC) = 0.14285714 BTC

This is the amount of bitcoin you would own. Changes in the dollar value of a bitcoin will not affect how much Bitcoin you own, it will only affect the dollar value of what you own.

then later on bitcoin drops to $6500, (...) what will happen if i add more money in my wallet then later bitcoin goes back to $7000, will i have more than $1000?

Yes.

To illustrate: let’s say you buy just $1 worth of bitcoin when the price is $6500/BTC:

($1)/($6500/BTC) = 0.00015384 BTC

Now add that to your original BTC balance:

0.14285714 BTC + 0.00015384 BTC = 0.14301098 BTC

Now, at a price of $7000/BTC:

(0.14301099 BTC)*($7000/BTC) = $1001.08