Question

In single-celled organisms, how growth and reproduction takes place, when growth is defined as increase in cell number or mass

Answer 1

In single-celled organisms, growth and reproduction takes place simultaneously.
Growth means increase in cell number or cell mass.
Reproduction means giving rise to offspring either by asexual or sexual means.

For instance, in bacteria(single-celled), growth and reproduction both occur simultaneously by the process of binary fission. Binary fission results in increase in cell number (1 divides into 2) and in a new progeny (2 daughter cells from 1 single mother cell) as well.
Hence, reproduction is synonymous with growth in single-celled organisms.