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

Source 1: Article 64, paragraph 1, Political Parties Act 2013.

Artículo 64 inciso 1°. – Las contribuciones privadas solo pueden acreditarse a favor de los partidos políticos.

Article 64, paragraph 1. – Private contributions may only be credited to political parties.

Source 2: Article 10, paragraph 1, Provisions for the Nomination of Non-Party Candidates in
the 2010 Legislative Elections

Artículo 10 inciso 1°.- El candidato no partidario podrá recibir donaciones de fuentes privadas, para lo cual abrirá una cuenta bancaria única a su nombre o del Grupo de Apoyo que lo respalda.

Article 10, paragraph 1. – Non-party candidates may receive donations from private sources, for which purpose
they shall open a single bank account in their name or in the name of the Support Group backing them.

Ley de Partidos: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/2E597B83-D1C9-4C91-A9C9-6EC8A44FBA0F.pdf 
Regulación candidaturas no partidarias: https://www.asamblea.gob.sv/sites/default/files/documents/decretos/45E06ECC-27EC-49EC-B5D5-5D29814BAD32.pdf  

This only applies to non-partisan candidates, as the regulations do not set limits. In the case of party candidates, it does not apply because the law establishes that only parties can receive contributions.
In fact, by not regulating the financing of party candidates, the law creates a loophole that is exploited in practice by candidates to raise funds without being accountable to their parties or the electoral authority for the origin and destination of the funds received.