I have Portcullis, Mishra's Factory, one creature, and one untapped land in play on my side and my opponent is about to begin their turn. I am able to activate Mishra's Factory and turn it into a creature during my opponent's Upkeep or Draw phase before their typical first chance to play a creature card (i.e. their Main phase) thus locking up the second creature spot on the battlefield courtesy of Portcullis, correct?
Yes, you can do that. Even better yet, you can hold back even more and do this in response to a creature spell your opponent plays and get that creature exiled. Your opponent is not hindered to play more creatures if there are already two out, neither are you, it's just that surplus creatures will get exiled until Portcullis leaves the battlefield.
Former Rules Advisor
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