Maybe because with that seed, every generated address is the subsequential?
like 1xxxxxxxa; 1xxxxxxxxb and so on?
I ask because I'm going to store some savings, and I'm scared that in a disaster event I'm going to lose everything...
You won't lose everything.
As long as you have those 12 words.
Using my 12 words I could calculate the first 100 addresses (along with their WIF private keys) of my wallet for you in about 1 minute using commands in a Python console WITHOUT even touching Electrum OR the internet.
It's all just math.
1 + 5 is always 6
Your 12 words will always generate the same addresses in the same order.
I'm new with electrum so this may be a stupid question but:
if I have received money from more than 100 addresses, does the seed changes or it is always the same?
The seed never changes unless you generate a new wallet. All addresses generated by Electrum can be recovered from the Seed. Imported addresses cannot be recovered.