How can I know how many times l'm charged monthly by Apple?

Unless you have yearly payments too, if you have 1 monthly subscription, once.

2 subscriptions, twice... and so on.

If you're sure that those charges come from Apple (check your bank statement very carefully, ignoring scam messages that might be addressed to your iPhone), start reading this support page:--> Subscriptions and Billing – Official Apple Support, and check each of the first 3-4 sections.

If you cannot find the information you were looking for, please contact iTunes Store Support Team directly and explain your problem, using this specific link:--> Apple Support

Regards

Giulio

Hello ~ Take a look here:

Subscriptions and Billing - Official Apple Support

~Katana-San~