What is the maximum income for a club with a 50000 seats stadium at the moment ?

Do we get any bonuses for playing in international cups ?

How is Ground Maintenance calculated ?

Sponsorship is limited by stadium size times 10, so a team with 50000 seats would receive, in the best case, 500k from sponsorship.
Merchandise is not related or limited by stadium size, it depends on players quality.
Ticket income will be attendance times 10, so again 500k if the stadium is full. For cup final round a neutral stadium with 100000 seats is used so your stadium has no influence on that.
There is no bonus for playing in international cups, the only benefit is the attendance tends to be always good and final match represent 1M from ticket income.
Ground Maintenance is a very simple formula right now, it will always be something around your stadium size, so for a 50000 stadium it will be around 50000.
Hope this helps.

Cheers!