Sometimes called "Numéro d'Identification Fiscale" (short form NIF) ("رقم التعريف الضريبي" in Arabic), and other times "Matricule fiscale".

I could not find any source about the structure or length. According to what I tested in the online search below, the length seems to be 8 digits, and if it is shorter you should prepend enough zeroes to get to 8 digits.

• Verify NIF online: https://impots.gov.mr:8080/DGI/ (found through https://finances.gov.mr/fr/node/22)

Examples:

• https://www.petrole.gov.mr/IMG/pdf/tableau_registre_petrolier_itie_2017.pdf

  20300059
  00409136
  90300075
  00455386
  00510669
  00510651
  00510677

• https://www.petrole.gov.mr/IMG/pdf/tableau_registre_petrolier_itie_2018.pdf

  20300059
  619429 (works if you prepend two zeroes)
  619437 (works if you prepend two zeroes)
  00409136
  90300075
  00455386
  00510669
  00510651
  00510677

• 20700455 from https://irshad.mr/avis-dattribution-provisoire-de-marche-pour-les-travaux-dinstallation-des-clotures-grillagees-de-100-hectares-des-aglc-dans-les-wilayas-de-lassaba-de-deux-hodhs-et-du-guidim-2/
• 00042218 from https://benteyba.mr/
• 11100448 from https://eps.mr/en/index.php?option=com_content&view=article&id=49&Itemid=54
• 00520510 from http://www.alem.mr/Contacts/
• 10600166 from https://www.agriculture.gov.mr/IMG/pdf/avis_d_attribution_definitive_-ma-2018.pdf
• 00066845 from https://beta.mr/docs/pdf/APM_HAPA.pdf
• 30500075 from https://somelec.mr/sites/default/files/Affichage%203414.pdf
• 00117549 from http://www.dtf.gov.mr/images/2019/CAMEC2019.pdf
• 70 401 906 from https://www.comavip.mr/quisommes.php
• http://www.cncmp.mr/IMG/docx/Publication_avis_attributions_definitives__ProLPRAF_MA_PND_MEDD_2015.docx

  30600091
  11100778
  10701982
  20400034

• https://www.bcm.mr/liste-des-prestataires-de-transfert-de-fonds-domestique-autorises-en-mauritanie

  20705520
  00554352
  00550608

• 01196641 from https://conecty.mr/
• https://filefr.ami.mr/pdf/6441.pdf

  3060009
  1110077

• 00060707 from http://www.afsa.mr/Manuel.pdf
• 00127456 from https://bca.mr/contact/
• 00407239 from http://www.transite.mr/
• https://www.fonctionpublique.gov.mr/IMG/pdf/pv06_2018.pdf

  00091769
  00055152
  10701354
  10701339
  10707336
  10713326
  10701495
  00194480
  00076299
  00516534
  10800042
  10706114

• 00066845 from https://www.commerce.gov.mr/fr/?q=node/1290
• 10709316 from https://www.sante.gov.mr/?p=2194
• 30200208 from https://www.somascir.mr/
• 10709951 from http://praps.mr/sites/default/files/document/avis%20d%27attributions%20a%20publier%20sur%20site%20praps%2010.pdf
• 30200174 from https://www.arpeco.mr/
• https://filefr.ami.mr/pdf/6225.pdf

  30700067
  30700113

• 20600117 from http://www.cncmp.mr/IMG/pdf/AVIS_ATTRIBUTION_DEFINITIFS_MAHADRAS_lot_2.pdf
• Several dozens at https://taazour.mr/fr/wp-content/uploads/2022/01/Repartition-constr-log.-soci.-DARI-1.pdf (attached here just in case)

  756924
  793182
  127258
  664169
  62869
  367771
  754028
  71266
  88021
  10717012
  560896
  10707955
  20705397
  656215
  286591
  412445
  10702865
  392050
  555649
  214148
  20602626
  733899
  767673
  10703574
  746909
  280685
  767889
  652487
  210518
  10713896
  352260
  291799
  291856
  292300
  292359
  291831
  346312
  869883
  865873
  687541
  10713201
  435297
  21103616
  21100901
  611145
  139071
  10600158
  190405
  752121
  397422
  800508
  506717
  831305
  550582
  39354
  20901545
  391276
  202705
  506915
  143065
  629642
  745224
  753673
  509828
  10708862
  226316
  520023
  20703590
  529529
  540377
  560938
  264853
  292391
  11100075
  233098
  327379
  60921
  85084
  619312
  827071
  498386
  701342
  10713102
  10707476
  733592
  629006
  20600125
  20601099
  751909
  820852
  10706544
  10712070
  346700
  64287
  621730
  10600174
  167866
  65425
  231720
  20702154
  355651
  10718358
  10715883
  518092
  10700513
  10602709
  20602808
  10721188
  10703103
  20601206
  501304
  20600083
  757815
  625947
  1060273
  69237
  39127
  33787
  661470
  293902
  801522
  20600760
  20704853
  434282
  726927
  10708987
  20701594
  267914
  188292
  10603400
  20704432
  765271
  74492
  20302865
  20600620
  309104
  10701776
  591388
  10704762
  525154
  421495
  687541
  662825
  843896
  793950
  778027
  10718820
  85563
  394932
  864900
  11101016
  83352
  20602782
  11108235
  30600323
  319681
  390286
  406892
  114157
  41731
  828608
  10700885
  837096
  20705470
  457085
  248880
  860072
  503821
  41100018
  777755
  856708
  41673
  10602410
  366682
  20602121
  747287
  714097
  372367
  11104234
  10718655
  735100
  40700018
  842815
  10720354
  791111
  752402
  21106429
  665158
  63040
  637579
  763326
  10600042
  20602477
  71722
  713537
  10718200
  11100935
  222083
  636670
  21100760
  58842
  20703400
  455113
  123802
  20704572
  498329
  10600620
  10701479
  748525
  566273
  248898
  283440
  856880
  751503
  20601081
  10721147
  837849
  10700604
  384685
  10601305
  135772
  10600166
  10723903
  10601933
  748970
  408120
  694018
  11107690
  251488
  11100182
  20602683
  89086
  20600141
  10700893
  746164
  412593
  380881
  393678
  10715974
  644880
  618702
  614149
  21101248
  20705181
  694018
  719773
  39354
  513598
  820613
  10701065
  854299
  20704176
  10600299
  20701560
  59493
  836353
  10715853
  276725
  118091
  525311
  11107518
  10722269
  756106
  789958
  10707708
  748525
  227900
  10900828
  139592
  513572
  10603079
  79411
  235796
  73320
  20704218