If you ever toyed with the idea that it would be cool to collect an entire Space Marine Chapter, or just like obscure
40k trivia, you may be interested in the chapters that field significantly fewer than the usual 1000 marines.
There's a number of such chapters, moreover, there's at least one which you could reasonably collect, build, and paint within a single day.
Obviously, there's also plenty of chapters with unknown current numbers, and you can always make up a chapter of your own - but let's assume you need a canon chapter with a canon chapter strength for additional bragging rights.
The list below is based on Lexicanum and
40k wiki. It is complete, i.e., it includes every chapter below 500 marines from the sites' respective Space Marine chapter listings. I'm noting down chapter name, strength, links, colours, and some commentary.
Sometimes there's a Citation Needed in a Wiki article - then, I add (assumed) at the chapter strength.
Angels Sanguine: 500 (assumed) - Blood Angels successors, colours are red and black, halved.
http://wh40k.lexicanum.com/wiki/Angels_Sanguine
Angels Revenant: few hundreds - Ultramarines successors, chapter destroyed by Necrons' Maynarkh Dynasty (the Forge World guys), but still has survivors. Colours are dark blue with bone white pauldrons, neat!
http://wh40k.lexicanum.com/wiki/Angels_Revenant
Angels of Wrath: one company + scouts, lacking in vehicles - Dark Angels successors who fight Book of Revelation-styled covert ops against the Ecclesiarchy. Colours unknown.
http://wh40k.lexicanum.com/wiki/Angels_of_Wrath
Astral Knights: 31 Marines and 1 Dreadnought - Imperial Fist successors. Colours are light silver and metallic blue, or blue, black, and yellow. Chapter destroyed while killing Necron World Engine, survivors fight to the death.
http://warhammer40k.wikia.com/wiki/Astral_Knights http://wh40k.lexicanum.com/wiki/Astral_Knights
Black Consuls: chapter destroyed, few survivors remaining - Ultramarines successors, Colours are black with yellow adornments and company-coloured pauldron trim.
http://wh40k.lexicanum.com/wiki/Black_Consuls
Crimson Fists: around 500 - Imperial Fists successors, colours are red and blue. Plenty of lore about these guys.
http://wh40k.lexicanum.com/wiki/Crimson_Fists
Consecrators: around 500 - Dark Angels successors, colours are black with some red details.
http://wh40k.lexicanum.com/wiki/Consecrators
Celestial Lions: fewer than 96 - Imperial Fist successors, colours are gold with blue pauldrons and helmets. Decimated by corrupt Ecclesiarchy. Valiant, good looking, African themed, and collectable within a short time - if only the Stormcast Eternals didn't use the same colours!
http://wh40k.lexicanum.com/wiki/Celestial_Lions
Flesh Tearers: around 400 - Blood Angels, but more savage. Colours are black and red.
http://wh40k.lexicanum.com/wiki/Flesh_Tearers
Iron Hearts: fewer than 200 (assumed) - nothing on them except a short story by Simon Jowett.
http://wh40k.lexicanum.com/wiki/Iron_Hearts
Invaders: 12 or fewer - Imperial Fists successors. Green armour with gold trim. Numbers reduced to double digits, but still fighting!
http://wh40k.lexicanum.com/wiki/Invaders
Inceptors: fewer than a 100 at the start of M41 - since then, they probably recovered. Ultramarines successors, green armour and gold trim. If they get Guilliman's new jump pack Primaris Marines, the confusion will be hilarious.
http://wh40k.lexicanum.com/wiki/Inceptors
Knights of Eternity: presumed destroyed, but maybe somebody survived? - killed by Tyranids, colours are red and white.
http://wh40k.lexicanum.com/wiki/Knights_of_Eternity
Legion of the Damned: fewer than 200 - do they count? Probably not. Everybody knows them.
http://wh40k.lexicanum.com/wiki/Legion_of_the_Damned
Lamenters: currently lost, strength probably below 200 or 400 - they're the best. Again, everybody knows them. If not, they should get to know them immediately.
http://wh40k.lexicanum.com/wiki/Lamenters (see
40k wiki for a different strength estimate:
http://warhammer40k.wikia.com/wiki/Lamenters)
Mantis Warriors: around 100 - White Scars successors. Colours are green and gold/yellow, they also have a neat camo. Their story is very similar to the one of the Lamenters.
http://warhammer40k.wikia.com/wiki/Mantis_Warriors
Marines Errant: more than 120 - Ultramarines successors. Colours are white and blue, halved. They are basically a
40k equivalent of the USS Enterprise. Very cool.
http://wh40k.lexicanum.com/wiki/Marines_Errant
Scythes of the Emperor: around 100 - Ultramarines successors. Colours are black and yellow. Fought alongside Lamenters, decimated by the Tyranids. They had it coming. A very cool army of them was made by GMM Studios, go check it out!
https://gmmstudios.blogspot.com/2014/03/memento-in-utero-adepticon-2014-army.html http://wh40k.lexicanum.com/wiki/Scythes_of_the_Emperor
If you have any info to add, you're welcome!
Note 1: notice that the numbers probably don't include scouts, techmarines, serfs, and other personnel - for details, see
https://www.dakkadakka.com/dakkaforum/posts/list/726277.page.
Note 2: I have no idea how to assume the number of vehicles, terminators, centurions, etc.
Note 3: when you boast you have a whole chapter completed, some may say that it's cheating when it's fewer than 1000 marines. So, you have been warned.