To ensure the customers are receiving QuickBooks emails; Solution 3 here suggests configuring your server to accept QuickBooks online mail server house names and IP addresses:

https://quickbooks.intuit.com/learn-support/en-uk/manage-customers/3-solutions-for-when-customers-ar...

Do I need to add all these (o1.sg.n.intuit.com, etc) as DNS TXT records to my DNS settings of my domain name? Or should they be MX records?

Why is there so many?

Why isn't there a QuickBooks SPF record?