What is a conservative number ​

Consecutive numbers are numbers that come one after the other.  like 3,4,5

I do not know......Consecutive number perhaps?

Sometimes, when people say ' A conservative number would be...'      they mean that the number is a close estimate without over- or under-estimating too much

I think you mean a " conserved number" or more accurately known as a " conserved quantity", such as "mass", "energy" "momentum"......etc. There is also a "conserved numbers" in physics, such as the spin af an electron, prtoton, neutron.....etc. This means that these numbers and quantities are "fixed". You cannot increase them or decrease them or change them. They are "fixed" by mother nature.

The expression conservative number can mean. cautious or not out of the norm. not far from the benchmark.