My Alta ski instructor is from NC. He's been teaching at Alta for almost 20 years. Said that Alta normally has about 6000 skiers on a busy day. The goal is to limit to about 4000. Presumably mainly by limiting sales of day tickets during a holiday period or on weekends. No reservations required for season passholders, which includes Ikon and MCP.
Once parking lots and roadside parking is full at Alta, taking the UTA bus is the only option. Unless know someone willing to drop you off and then drive back down to SLC. There will be someone actively monitoring uphill traffic at the base of the canyon. Have been a couple days that parking was full by 9:30am, with social media warnings by 8:00am that parking was "limited" and expected to be full by 10:00am.
2020-21: JH/GT, Alta, Massanutten. Planned: Indy for Cat, CV; Ikon in Colorado plus Monarch, Wolf Creek.