myriad = 10,000
†רִבּוֹ S7239 TWOT2099e GK8052, רִבּוֹא S7239 TWOT2099e GK8052 n.f. Ezr 2:64 ten thousand, myriad (later (Aramaizing) synonym of foregoing; perhaps for Aramaic רִבּוּ(ת), Kö. 1, 221 f., but Palm. רבו Lzb 367);—abs. רִבּוֹ 1 Ch 29:7(); Jon 4:11 (? cstr.), + cstr. Ho 8:12 Kt (v. infr.); רִבּוֹא Ezr 2:64 = Ne 7:66; Ne 7:71 (van d. H. v 72); du. רִבֹּתַיִם ψ 68:18; pl. רִבּוֹת Ne 7:70 (van d. H. v 71), רִבֹּאוֹת Ezr 2:69; Dn 11:12;—ten thousand; of pers., 4 × 10,000 + 2000 + 3 × 100 + 60 Ezr 2:64 = Ne 7:66; 12 × 10,000 + n. Jon 4:11; indef. Dn 11:12 he shall cast down myriads; read רִבּוֹת also v 41 (for MT רַבּוֹת) Kmp Bev Behrm Marti Prince; of things, especially units of value, n. + 10,000 1 Ch 29:7, n. + 6 × 10,000 + 1000 Ezr 2:69 || n. + 2 × 10,000 Ne 7:70, 71 (v. supr.); 10,000 + 8 × 1000 + n. 1 Ch 29:7; indef. רֶכֶב אֱלֹהִים רִבֹּתַיִם ψ 68:18 (|| אלפים);—רִבּוֹ תוֹרָתִי Ho 8:12 (Kt; 10,000 [precepts] of my instruction) would be only early instance, v. רֹב.
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 914). Oxford: Clarendon Press.