What does Article 87 of the AI Act say about reporting of infringements and protection of reporting persons?

You determine this yourself

• The trigger is a report of an infringement of this Regulation, whatever the risk class of the system: a report about an AI system outside the high-risk category is covered just as much. The protection itself is not unconditional. It comes from Directive (EU) 2019/1937, which in Article 4 requires the person reporting to have obtained the information in a work-related context, and in Article 6(1)(a) requires reasonable grounds to believe that what was reported was true and fell within the scope of that Directive.
• Article 87 creates no channel requirement. That requirement comes from Article 8 of Directive (EU) 2019/1937. Paragraph 1 places it on legal entities in the private and the public sector; paragraph 3 limits paragraph 1 in the private sector to entities with 50 or more workers. That threshold is not general, however. Paragraph 4 provides that the threshold in paragraph 3 shall not apply to entities falling within the scope of the Union acts referred to in Parts I.B and II of the Annex to that Directive, which cover financial services, anti-money laundering and transport safety among others. Paragraph 7 allows a Member State, following a risk assessment, to require entities with fewer than 50 workers as well. Paragraph 9 applies paragraph 1 to all legal entities in the public sector, with the option for a Member State to exempt municipalities under 10 000 inhabitants and other small public entities. Below fifty workers there is therefore not simply no channel requirement: it depends on the sector you fall in and on what your Member State has decided. The right to report and the protection of the person reporting exist in any event, through the external route of Article 10 of that Directive.
• The material scope of Directive (EU) 2019/1937 runs through Article 2(1)(a), which refers to the Union acts listed in the Annex to that Directive. Regulation (EU) 2024/1689 was not added to that Annex: it makes the Directive applicable directly, in Article 87. National transposition law that ties its own scope to that same Annex, such as the Dutch Wet bescherming klokkenluiders, may therefore lag behind the Regulation. Whether a report about an AI system falls under national law as a result is not settled.