The number of Clanrat units in the army restricts the maximum number of multiples of any other unit that can be included in the army. For example, if you have one unit of Clanrats, you can include one unit of Globadiers, one of Stormvermin, one of Gutter Runners, etc. If you have two units of Clanrats then you can include up to two units of Globadiers, up to two units of Gutter Runners, up to two Warp-lightning Cannons, etc. All normal restrictions still apply on top of this.