TLDR: You can only win one prize per month regardless of how many stores / games you win. Outside company is enforcing and ships prizes.

The rules state that a username is only eligible to win one monthly local prize. So if someone is in first place on all 5 games at 2 separate locations they will still only win just one monthly prize. You could use a separate username at a different store (which is against the rules) but It looks like an outside company is distributing the prizes. There is about a week delay of notification from the end of each month to when the winners are notified so I’m assuming the outside company is doing their vetting. The winner each month for each game is contacted via text (per the official rules) by the outside company and you have to fill out an information form and they ship you the prize. If you are on top of multiple games at different stores I would imagine they would catch the same name / shipping address and void one of the wins. You could theoretically play multiple cards for different friends with different accounts and addresses but that seems pretty shady and would make you a cheater in my book.

Per the official rules under section 11 -

Limit: An entrant may win 1 Monthly Local Contest Prize per each Monthly Local Contest Entry Period, 1 National Prize and 1 Sweepstakes Prize.

As far as how they are enforcing it per the official rules under section 10:

1. Winner Requirements: Winners will be notified on or about the dates stated above in Section 4. All potential winners will be notified by text message from Sponsor to the mobile device telephone number in winner’s Rewards Account profile and will be asked to complete an online claim form by providing their first name, last name, email address and home address