This expression, which I have never seen before in English, turns out to be literally translated from Colombian Spanish. The only English text in which I can find it is a translation of Colombia's copyright law, Law 44 of 5 February 1993. The sentence you have quoted comes from this translation. Here it is in English:
http://www.unesco.org/fileadmin/MULTIMEDIA/HQ/CLT/diversity/...

Your sentence is in Article 19. And here is the Spanish version, in which the sentence appears as follows:

"ARTICULO 19. El Comité de Vigilancia estará integrado por tres (3) miembros principales y tres (3) suplentes numéricos, quienes deberán ser miembros de la asociación. Sus atribuciones y funciones se precisaran en los estatutos."
http://www.wipo.int/wipolex/es/text.jsp?file_id=126030

So "numerical alternates" is a literal translation of "suplentes numéricos". In Colombia there are two kinds of alternates: "personal" and "numérico". This mainly arises in company law in relation to boards of directors. The difference is that each "personal" alternate is an alternate for a particular member of the committee. With three members and three alternates, if a member is missing the specific personal alternate corresponding to that member will act in his/her place. But with "numerical" alternates, the three alternates form a single "pool", with an order of precedence: there is a first alternate, a second and a third. If one member is missing, whichever member it is, the first "numerical alternate" will replace him/her. So the second and third numerical alternates will only act if two or three members are missing.

Here, for those who can read Spanish, is an explanation of these Colombian terms. See paragraphs 4-6.
http://www.accounter.co/normatividad/oficios/oficio-220-0438...

--------------------------------------------------
Note added at 18 hrs (2018-01-18 07:16:25 GMT)
--------------------------------------------------

"Numerical alternates" is not used in English and was not a good choice to translate "suplentes numéricos". So what should they be called in English? I have just realised that I myself answered a question on this Spanish term here in 2015. I found that "general alternate" and "non-specific alternate" are used in genuine EN sources:
https://www.proz.com/kudoz/spanish_to_english/management/581...