How is the typical OEM licensing payment term?
For example if another party want to buy 50 licenses, do they need to pay in full amount ? or some percentage in initial agreement and the remaining after a few months etc ?
One more thing, is it normal to ask another party to keep buying for certain (minimum) licenses for every year ? If at initial agreement they agree to buy 50 licenses, can I ask them to repeat it every year?
I don't think there is a "normal" for these kinds of things. Contracts vary greatly. If it makes business sense, then you can put it in your contract.
For payment, it is best to get payment up front if you can as customers sometimes stop paying, but you can certainly do an installment plan as well.
You can also do a multi-year contract where the customer has to buy X licenses for each year of the contract. If you are the licensor, you should have the contract auto renew unless notice of nonrenewal is given 30-60 days before the end.