Is this brainwallet approach safe enough? Let's suppose I derive a private-public keypair from a passphrase like this : 

long_and_complex_enough_but_yet_easy_to_remember_passphrase

Now I wish to generate 100 more keypairs using same passphrase, so I append consecutive numbers : 

long_and_complex_enough_but_yet_easy_to_remember_passphrase_001
long_and_complex_enough_but_yet_easy_to_remember_passphrase_002
long_and_complex_enough_but_yet_easy_to_remember_passphrase_003
...
long_and_complex_enough_but_yet_easy_to_remember_passphrase_100

Is this safe enough to do, as long as the initial passphrase is enough complex one, or maybe the consecutive numbers add any vulnerability ?
Appending numbers doesn't open you up to any particular vulnerability, but using any human derived private key is an extremely poor idea. Markov chains are extraordinarily good at attacking passphrases, which is counter intuitive when most people assume an exhaustive search character by character.