There is more than one plural suffix. Some words use suffixes and stem change together. Swahili loanwords which are really integrated in Kinubi vocabulary have a plural prefix. There are also some (rare) traces of dual.
Source: Luffin 2005: 147
Source: Luffin 2005: 138
Source: Luffin 2005: 141
Source: Luffin 2005: 144
Source: Luffin 2005: 149
Source: Luffin 2005: 150