Well, I don't know myself, I'm only basing at what Mikhail Baryatynski wrote (quite a known expert on Soviet armour). It's very probable, that capacity was 20, while the gamers may refer to actual number of soldiers carried, which depended on organization (it was sure more comfortable for less soldiers). For example, in the Czechoslovak Army, a modified OT-62 carried crew of 2 and 14 soldiers (two sections) plus platoon's commander (sketches at
http://forum.valka.cz/viewtopic.php/t/53321 )
You've made me to search an original Soviet military manual of BTR-50, which I've just downloaded, and which says on p.4: "each APC of both variants enables carrying or ferrying [ie. on water] of 20 troops with full combat equipment or 2000 kg of different loads". In a troop compartment there were two 4-seat benches along sides, upon tracks and three longitudinal removable benches along the compartment. It is BTW interesting, that BTR-50P could carry a 57-85 gun or 107 mm RG or 3 mortars or GAZ-69 jeep atop.
Hope this info helps.
PS. BTR-60 indeed had capacity reduced from 14 (P) to 12 (PA) and only 8 in PB - it might need changes as well (possibly 9, if we include commander sitting next to driver).
Regards
Michal