You should always have at least 2-4 key contacts on the customer’s team. It’s a massive risk if you only have one point of contact (what if they go on vacation, what if they leave the company, etc). You definitely never want to have all your eggs in one basket. The more you do that the bigger risk you run of having to start over.
Whenever I do an onboarding I always request that sales provides 3 points of contact from the customer. One is the executive sponsor (person who signed the contract), the app owner (person who I will work the most with), and a power user/champion (an end user that will champion the adoption of the product).
If you have those three invited, your chance of completing your onboarding on time goes up significantly.