You have to go back to Numbers 16 and read that these Israelite's that complained in Numbers 17:12-13 were arguing in Numbers 16 why certain Israelite's made themselves more special, and are the only ones that can approach the Tabernacle of Congregation. They were insinuating that Moses and his brother Arron made up these laws and so Moses had to prove to them that the LORD made all the decisions that Moses enforces, and not Moses or Arron decided them themselves.
So Moses ordered that each Prince (leader) of every 12 Tribes to appear before the Tabernacle with Censors "the Lord will show who is Holy and who he will allow to come near unto Him..." see Numbers 16:5 (K.J.).
And in Numbers 16:21 the LORD was telling Moses in front of all these complainers to step away from the complainers so that the LORD could destroy them all.
Instead Moses pleaded with the LORD and explained to the complainers that he would prove it was not him or Arron making the decisions, it was the LORD. So Moses told the people that if the LORD miraculously kill's all the complainers, then they will know that its not Moses making the decisions on who is allowed to approach the Tabernacle of Congregation, the Tent of the LORD that held the Ark of Covenant, see 16:28-34, (K.J.)
So in Numbers 17:12,13 the rest of the Israelite's that witnessed the complainers being sucked into the ground by the wrath of God were "JUST ACKNOWLEDGING" in a bummed out type of way that they understand clearly now who the LORD had chosen to approach the Tabernacle of Congregation and do the works therein. And they seemed to be also acknowledging that if one of their families or Tribesmen trespass it could cause others to die with them?
Then in Numbers 18 the LORD clarifies who is allowed to approach the Tabernacle and "not die" and their duties. And the LORD lets the rest of the Israelite's in Numbers 18:22 know that His Commandment is that if anyone other than those the LORD allowed, approaches the Tabernacle or anything that is hallowed onto the LORD, they will surely die.
Another example of Numbers 17:12,13 where the Israelite's acknowledged OUT LOUD what they learned if they approach the Tabernacle, (that they will die) can be found at Joshua 24:19-24 where at verses 21, 22, and 24 the Israelite's repeated OUT LOUD what they were acknowledging about the LORD's Commandments that was then being explained by Joshua (as Moses did about the duties of Arron and the Levite's in Numbers 16 and 17). Joshua 24: (V.21, "we will serve the LORD.") (V.22, "We are witnesses.") (V.24, "The LORD our God we will serve, and his voice will we obey)
So, in Numbers 17:12,13 except for the Tribe of Levite, the Israelite's are merely acknowledging out loud to Moses and the LORD that they now understand and except that the LORD chose only the Tribe of Levite to approach the Tabernacle of Congregation. And that if any other Israelite approaches the Tabernacle, or anything hallowed, they and their families will die!