Compositions and methods are provided for measuring population size for a plurality of clonal cell populations in the same individual, e.g., for measuring tumor size for a plurality of clonally independent tumors within the same individual. A subject method can include: (a) contacting an individual with a plurality of cell markers that are heritable and distinguishable from one another, to generate a plurality of distinguishable lineages of heritably marked cells; (b) after sufficient time has passed for the heritably marked cells to undergo at least one round of division, detecting and measuring quantities of at least two of the plurality of cell markers present in the contacted tissue, thereby generating a set of measured values; and (c) using the set of measured values to calculate the number of heritably marked cells that are present (e.g., for at least two of the distinguishable lineages of heritably marked cells).