It is better to submit more certificates than fewer. However, it depends on whether the purpose is confirming your stay in Iceland or your Icelandic skills.

If your stay in Iceland is being confirmed, the certificate must confirm the period during which you were enrolled at that school.

If your Icelandic skills are being confirmed, the certificate must indicate that you have, for example, been a student at an Icelandic elementary school or meet other criteria for an exemption from the Icelandic language test.