Do not make instance data thread safe by default, it's a thing we know, but do you know, or have you tried, if a single instance of IClientSessionHandle can be used in tasks executing in parallel?

There is a 3rd party library that exposes the method, which updates a single record in the collection called Customers:

Task UpdateCustomer(IClientSessionHande handle, CustomerUpdate customer);

Can this UpdateCustomer method be used with multiple tasks executing in parallel?

Task UpdateMultipleCustomers(IClientSessionHande handle, IEnumrable<CustomerUpdate> customers)
{
     return Task.WhenAll(customers
          .Select(customer => UpdateCustomer(handle, customer))
     );
}