A DMARC check begins by fetching all TXT records published at _dmarc.<domain> that start with v=DMARC1. In case you want to know how many DMARC records you can have on a single domain, the only correct answer is ‘ONE’. A domain must not have more than one DMARC record if you want the DMARC processing to work successfully on that domain. 

DMARC processing stops if no such record is found as well as if multiple such records are found.

For instance, you have two TXT records published at _dmarc.example.com:

So, if you have more than one DMARC record on a single domain, your intended DMARC policy will fail to apply and DMARC reporting may not function correctly or consistently

For rectifying such issues, it is essential to keep just one valid DMARC record on your domain and remove all others. 

When you secure your domain with TDMARC, it provides you with a unique CNAME. You must remove all existing DMARC TXT records before adding the TDMARC-provided CNAME, ensuring only one DMARC policy is resolved.