Short answer. Under Section 5 of RA 9048, you must submit at least two public or private documents showing the correct entry on which the correction will be based. These come on top of a certified copy of the record containing the wrong entry, and the registrar may ask for other documents he considers relevant.
What the law says
At least two (2) public or private documents showing the correct entry or entries upon which the correction or change shall be based
RA 9048, Section 5 — Form Contents And Publication. Read the full provision →
At least two documents showing the correct entry
The heart of the answer is a minimum, not a fixed number. Section 5 of RA 9048 requires At least two (2) public or private documents showing the correct entry or entries upon which the correction or change shall be based. "At least two" means two is the floor — you may submit more, and in a doubtful case more is wiser, but you cannot rely on a single document. They may be public documents (such as government-issued records) or private ones, so long as they credibly establish what the correct entry should be.
These are in addition to the erroneous record
The two proof documents are not the only papers required. Section 5 first calls for a certified true machine copy of the certificate or of the page of the registry book containing the entry or entries sought to be corrected. That establishes what the record currently says. The at-least-two documents then establish what it should say instead. So the registrar is comparing the flawed official record against independent evidence of the correct fact.
The registrar can ask for more
The two-document minimum does not cap what may be needed. The same section allows other documents which the petitioner or the city or municipal civil registrar or the consul general may consider relevant and necessary for approval. In practice this means the registrar can require additional proof if two documents do not convincingly settle the correct entry. The safest approach is to gather every reliable record that supports your position — school records, identification, baptismal or medical records, and the like — rather than stopping at the bare minimum, especially where the error touches something the registrar may scrutinise closely.
Change of first name follows the same proof rule
If your petition is for a change of first name or nickname rather than a clerical correction, Section 5 says it must likewise be supported with the documents mentioned above — so the at-least-two-documents requirement carries over. A first-name change adds further steps, including newspaper publication and a certification from law enforcement that you have no pending case or criminal record. The number of proof documents, though, starts from the same floor of two. Whatever the petition type, assembling solid documentary proof of the correct entry is what gives the registrar a basis to grant the correction under the administrative process.
A later law expanded this process
This administrative correction process comes from Republic Act No. 9048. In 2012, Republic Act No. 10172 amended Republic Act No. 9048 to add two more errors — a wrong day or month of birth, and a wrong entry of sex — to this same civil-registrar route, without a court order. The procedure and documents above still apply; Republic Act No. 10172 widened what can be fixed, not how it works.