Cyanobacteria is a whole phylum of bacteria rather than a particular species so the number will vary depending on the individual species. For bacteria in general, this is further complicated by the fact that when nutrients are limited or the temperature is below the optimal growth temperature, bacteria continue to grow but more slowly.
For example, E. coli bacteria in the laboratory can divide as quickly as once every 20 minutes. Out in the real world the division rate is probably more like once per day.
In the case of photosynthetic bacteria like cyanobacteria the main factors controlling replication rate will be mainly be temperature and light intensity which will be a function of depth, latitude, season, cloud cover, etc. The amount of dissolved minerals and carbon dioxide may also play a role.
how can we control growth of cynobacteria by a biological copetitive