Just wondering, let's say I do 1 backup and then receive bitcoin in 1200 different addresses, then my main wallet.dat file gets lost. If I use that 1 backup file I made way back then there is absolutely no way to recover coins send to the addresses after the 1000th address. Is that correct?
No, it is not correct.
If you
receive bitcoin in 1200 different addresses, when you recover using the backup, it will find all the transactions in the first 1000, then start generating "new" addresses which, because of the nature of HD wallets, will be identical to the original 200 "missing" addresses... and thus it will find all 1200 bitcoin addresses... it will then top up the keypool again with more addresses until it has 1000 unused.
As TryNinja and LoyceV have indicated, even if you didn't receive any coins on the first 1000 addresses... you will still be able to recover the funds received, by manually forcing the wallet to generate the "missing" addresses. Again, because of the nature of HD wallets, it will always generate the same group of addresses as it always starts from the same starting point and follows the same path when generating the keypool.
In short, you will be able to recover ALL your
funds. What you will not be able to recover will be any custom labels that you manually added to addresses after the backup was taken (as the backup will not contain these labels).