Hello KChan,
If you have a paid registration service (which for .COM is $16/yr), then you will have DNS services attached to that are part of the package.
No other fee's unless you want to purchase an Add-on such as:
- Host Monitoring/Failover records
- Tertiary Nameservers
- Backup MX
- Outbound SMTP
- Then you'll need to purchase Credits to be billed monthly for those above services.
We don't have limits per se, but the control panel will become slower to load depending on the number of records that it needs to display. Once you go above 1000 records, things could be sluggish.
For queries DNS query, it's 250k / mo for Free DNS members and Unlimited for Paid DNS/Registrations.
If you have any further questions or concerns, let me know.