Is patreon a monthly subscription? and does it automatically deduct money?
it depends on how the creator set their billing cycle. Typically it is the 1st day of each month but some creators have enabled an initiation fee meaning you get charged for joining and then get charged on the 1st of the month.
For Andy's Patreon it has an initiation fee and default cycling so I recommend joining at the start of a new month instead of the end of one so you don't get double charged.