You can request one and only after it is done can you request another. If it takes a month then it takes a month. If it is a request for a real car from this year then don't bother since they don't allow it plus its most likely already on the crews to-do list (or will be on NNR or some other site)so just wait. But if its for a fictional then ok. And the mod doesn't matter either its one at a time period.