@OP there are no rules, you can send to any user and any amount you want, but the maximum is 50 Merits.
The maximum amount is 50 merit for one merit transaction and for total amount of merit transaction between 2 users. This cap is applied for merit sources too, and one merit source can not send more than 50 merit to one user (like a newbie) in 30 days.
In a summary, merit cap is:
50 merit in 30 days for one user.
50 merit for one transaction.