This question has very limited information so will do my best based on it. There are multiple approaches - one of them outline below

I assume this is the market in one country

1. Estimate country population

2. Estimate number of couples (in relationship or married) - here you can take some broad assumptions based on observations - e.g., 60% of population above 25 is in some form of a committed couples relationship

3. Estimate demand for those couples for trips dedicated to the types of vacations you are estimating (will have to deduce based on your best estimates). Let's say 10% of couples would consider it

4. Use 10% to get potential market size. Then account for factors such as costs, location, duration, appeal etc. to get to actual market size - say 4%

5. Estimate avg. price for such vacation (usually $2k and up)

Now you have one way to calculate it

Note that you can add complications such as accounting for people coming from other countries, no kids vacations vs ones with kids, specialized markets e.g., those targeting younger couples etc.

Hope this helps

(edited)