There are several ways, if is a coop game you can get the number of rounds with this
int entity = FindEntityByClassname(-1, "terror_player_manager");
if (entity != -1)
int missionWipes = GetEntProp(entity, Prop_Send, "m_missionWipes");
PrintToChatAll("missionWipes %i", missionWipes);
You could also track by hooking the round_start event I think.
Probably is possible to get it through vscript aswell but I don't have the knowledge.
This kind of question is kinda unrelated to the OP topic so maybe its better to ask in the scripting area.