This is Iris from Social Care at Shopify, thank you for reaching out to us.
Regarding your question, the easiest way to limit the number of products is through a third-party app, here some recommendations:
You can use one of these apps to limit the number of products, but this wouldn't limit it based on the customer.
There is another app that can help to lock down some products that don't have maximum restrictions and allow customers that have access to buy as many as they like, to view more details please review the app [Locksmith].
For future reference, if these apps don't meet your expectations you can also try using Shopify Experts. Shopify Experts are third-party developers that can help you achieve your goals by helping you with designing, coding, among others. All of the experts approved by us are on the [Experts hub].
I hope you find this answer helpful if you have any further concern, please let me know.
Have a great day!