I have come across the term spp. or sp. when researching spiders at point. What does this exactly mean?
I believe it is used in nomenclature to specify the number of unknown species- say, I saw one gull I didn’t know the species of; I say Sp. If I see a group of gulls of multiple species, I say Spp., as it acts as the plural for a group of species.
Guess I can’t simply say “yep” 
So funny that the words species (singular) and species (plural) are the same and yet the abbreviations are different… And why two p’s? Why not sps.? 
Edit: I tried to Google who came up with these abbreviations or where they come from, but I couldn’t phrase the question in a way Google would understand
It’s the same as page vs pages: p. vs pp. Wikipedia says the second p. in spp. stands for pluralis.
My guess is that SPP → SPecies Plural
I don’t think I’ve seen anyone use ‘sp.’ regularly. It’s just the short form for a singular species.
‘Spp.’ Is the abbreviation for a group of species, usually within a genus. As in: Crataegus spp., or Amelanchier spp., Salix spp..
oh and there’s ssp. just to confuse the unwary (for ‘subspecies’)
The extra ‘p’ does not stand for “plural”; in Latin, abbreviations are pluralized by doubling a letter.
So, a sparrow species could be abbreviated as " sp. sp.".
I see (and use) it all the time. If I know it’s in genus Melaleuca but don’t know which one, then I write “Melaleuca sp.” iNat just happens to use a display form that avoids this.
… and of course the plural “sspp.” However, the accepted plural for “var.” (variety) seems to be “vars.” It’s all as consistent as everyday English.
well, I use sp. often - especially when talking about Carex…
We normally use just sp. for these. “Crataegus sp.” means some hawthorne, not identidied to species, only to genus.
I recommend subsp. (subspp.) rather than ssp. (sspp.) because of the visual similarity of spp. and ssp.
Amongst UK entomologists I’ve found it’s even quite common to hear people say ‘sp.’ not just write it. It’s a bit joking, usually despairing of something being identifiable, but I have heard plenty of people say out loud things like: “That’ll be another 'Syrphus spuh!”
I use “spuh” often to mean we’ve agreed it’s in a particulat genus but can’t get it better defined. I suppose one could say “subspuh” if you can’t decide what subspecies it is.
or just say it like a snake… ssssssssssss-puh!
Are they? In the latin based Spanish (and Galician) the singular is “especie” but the plural is “especies”.
Next let’s go ahead and question the widespread Germanic usage of “spec”
Wikipedia is simply incorrect about that. Latin duplicates a letter in an abbreviation to indicate plural. E.g. the plural of q.v. (quode vide) is qq.v. (quae vide), and the plural of f. (folio) is ff. (folium).
English is somewhat unusual among world languages in that it doesn’t use duplication of words, morphemes, or syllables grammatically. WALS says only about 15% of languages in their sample don’t. And many of the other languages which don’t are Indo-European, thus even multilingual English speakers are often surprised to discover that such a large percentage of languages use reduplication systematically.
The notion that spp. stands for “species pluralis” has the hallmarks of a guess misunderstanding that’s been repeated so often it’s now reported as fact. If it really stood for “species pluralis” it would be written sp.p., because people using Latin abbreviations in English are really pedantic about that kind of thing.
Edit: If you ask someone what sp. and spp. stand for the reply might be “They stand for species, singular, and species, plural.” Latin has the same problem, where the singular and plural forms of ‘species’ are identical, so if you ask someone in Latin what spp. stands for, they would answer “species pluralis”. But they assume you know about the duplicated-letter convention for abbreviations of plurals, so what they’re saying is “It stands for species” and then clarifying that it’s the plural form of the word. But that’s really easy to misunderstand if you don’t already know how Latin handles plural words in abbreviations and that the Latin word ‘pluralis’ can mean either ‘multiple’ or ‘(grammatical) plural’.