20. Is there a limit on the amount a candidate can contribute to their own election campaign?

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

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

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

Source 2: Article 10, paragraph 4, Provisions for the Nomination of Non-Party Candidates in
the 2010 Legislative Elections

Artículo 10 inciso 4°.- El aporte patrimonial de los candidatos no partidarios, entre propietario y suplente, no podrá exceder del cincuenta por ciento del total del presupuesto previsto para el desarrollo de la campaña electoral respectiva.

Article 10, paragraph 4. – The financial contribution of non-party candidates, between the owner and the substitute, may not exceed fifty percent of the total budget allocated for the respective electoral campaign.

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-party candidates, as the regulation sets 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.