TechCrunch notes that the beta testing will be limited. A detail which is echoed from Nintendo's tweet over on the official Mario Kart Tour twitter page.
That same tweet also mentions the time line of when players can expect to see the beta test begin.
The Mario Kart Tour multiplayer beta will start in December
Nintendo doesn't give out very much information on exact timing. Meaning there's no mention of a day or time of day. It does however list the month when the beta test will start. Which is December.
That's a month away at the very least. Nintendo could choose to release the beta at the very beginning of the month. It may also choose to release it towards the middle or the end.
The only thing to do at the moment for anyone interested is to wait for more information. The good news is that Nintendo has stated it will share more details on the beta and it's availability soon. One thing to keep in mind is that the beta will be limited in more ways than one.
The beta will be available only to subscribers of the Gold Pass
For the time being Nintendo says the beta will be limited to Gold Pass subscribers. This move could be for multiple reasons. For example it may want to weed out anyone that isn't extremely invested in this game.
It likely wants multiplayer to be a big feature, and having players who love the game enough to buy the Gold Pass be the pool of testers means those players would pay extra special attention to it.
It's also likely that this is a move to help drive sales of the Gold Pass.
In fact it's a pretty big possibility, seeing as multiplayer is a feature people were hoping for prior to the game's launch.
That being said, for Nintendo's part it's more important for it to make sure everything with the real-time multiplayer in Mario Kart Tour is polished before it releases it to all players. And, limiting the beta access to Gold Pass subscribers for now is a good way to ensure that proper stress testing is getting done.
— Mario Kart Tour (@mariokarttourEN) November 1, 2019