Obviously more words would make it more secure.
Hackers take the advantages of the mistakes made by users
If your seed phrase is generated on an air-gapped computer, there's nothing a hacker can do.
As stated by DaveF, A 12-word seed phrase provides enough entropy.
Anyway, if you want your seed phrase has more words for any reason, you can ask electrum to generate a 24-word seed phrase. (As humans may not be good at generating random words, I think that's a better option.)
To do so, go to console and use the following command.
make_seed(256)
The 12 words are 128 bits of entropy, which is considered more then enough.
A 12 word seed phrase generated by electrum provides 132 bits of entropy.