It's a concern, but fortunately if cattle population stays constant, their portion of the atmospheric resident methane will also remain constant, because methane degradation will keep pace with emissions.
Compare this to cars. Even if the number of cars remains constant, their contribution to the C02 pool grows every day.
So we should avoid increasing cattle population 100x, but we wouldn't want to do that anyway because we'd have nowhere to put them, and I certainly wouldn't support deforestation.
World population is largely a strayman, it's mostly about rising standards of living. Animal consumption per capita increases drastically as poverty dissipates, and sharply declines as people get educated about health and the environment. It's a race between those two pressures that determines the cattle population.
Projections are that it will increase very, very drastically as middle eastern and african countries soon escape the poverty line
