To answer you have to come up with some estimate of population growth rate, let's say in percent per year.
Right now most industrialized nations in the world have negative growth rates if you don't count immigration, or they will soon have negative growth rates given their population distribution. (Namely, too few young people.)
Estimating population growth historically is difficult because we don't have very good numbers for population more than 300 years or so ago. In that time, growth rates appear to have hovered around 0.5% per year. Such a growth rate gives plausible population numbers for most of history. Of course there have been ups and downs.
(From a creationist perspective, if Noah's Flood was approximately 4500 years ago, then to get from 8 people to 8 billion in 4500 years would require a growth rate of about 0.46%, which is consistent with observed rates.)
The fastest growing countries today have growth rates of around 3% to 4%. So such numbers are possible.
A lot depends on how difficult conditions are. If we assume that after the disaster the environment is mostly undamaged, that the world is wide open and empty, that modern technology can be rebuilt much more quickly than it took to develop it the first time, and that there are stored supplies to get the people (or whatever) through the difficult starting times ... growth rates could easily be very high, 5% or 6% or more. If we make opposite assumptions, that conditions are hard and people are struggling to survive, the growth rate could be very low.
In general, the formula is going to be: x * r^500 = 1e9, where x is the initial population and r is the growth rate. Solving for x gives 1e9/r^500.
At 5%, to reach a billion people in 500 years requires an initial population of less than 1 person mathematically, so realistically 2 people. Of course if you start with only 2 people that creates problems of genetic diversity, especially if whatever catastrophe killed everyone off resulted in chromosomal damage.
At 4%, 3 people. Again, genetic diversity becomes an issue.
3% -> 381 (as Nepe says), say 300 to 400
2% -> 50,108, say about 50,000
1% -> 6,907,376, say about 7 million
0.5% -> 82,597,922, or about 80 million
If there was such a catastrophe on Earth in the next century or so and the population was reduced to hundreds, there could be problems with genetic diversity. There'd be a lot of inbreeding, which would probably result in high death rates for the first few generations until the worst mutations were bred out of the system. That might mean a lower growth rate for a century or so. Or it might simply mean that people have more children to make up for the high child mortality rate.