As other reporters have written, The Shooter could have broken his weapons down to carry them in suitcases, hidden then in a golf bag as "just other clubs with those cute knitted head covers," in a tennis racket bag, or in a duffel bag....or any other bag. He was there for days....and no one would think twice he he came and went with a golf club bag once or twice a day. 
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And some people always - always - put out the "Do Not Disturb" sign even when they are not in the room. They don't want to come back to their room to shower/rest and find the maid there cleaning. 
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We are talking about a Hotel/Casino, not your parent's house, where just the vast number of people coming and going all the time at all hours really isn't that unusual. 
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So, no, it is not unthinkable that The Shooter eventually got 23 weapons into his room and no one noticed....at all....especially if a maid never went in to clean.