18. Is there a limit on the amount a donor can contribute to a candidate?

Código Electoral de la República de Panamá
Artículo 248: 
Ninguna nómina de candidatos a cargo de elección popular podrá recibir de una sola fuente contribuciones privadas que excedan los siguientes porcentajes sobre sus respectivos topes de gastos:
1. Las presidenciales, 3 %.
2. Las de diputados, incluyendo a los del Parlamento Centroamericano, los alcaldes, concejales y representantes de corregimiento, 25 %.
Para aplicar este tope, la nómina restará lo que financie con recursos propios.

Article 248: 
No list of candidates for elective office may receive from a single source private contributions that exceed the following percentages of their respective expenditure ceilings:
1. Presidential candidates, 3%.
2. Those of deputies, including those of the Central American Parliament, mayors, councilmen, and town council representatives, 25%.
To apply this ceiling, the list of candidates will subtract what it finances with its own resources.

Unofficial English translation.

The Electoral Code refers to this donations when they are coming from a single source (See articles 245 and 248)