I am new to cooking. I have guests coming over. How do I know how much food to cook? I usually cook for 1 person and there will be 5 of us in total.
It can be difficult to estimate how much food you should cook, as the appetite of different people varies according to their age, body weight, physical activity, habits and genetics.

I suggest you follow a written recipe you like, with exact ingredient amounts for one person, and multiply the ingredient quantities by six times so you produce six portions. If one of your guests has a bit of an appetite they will be quite satisfied with the amount provided, and if not, you have one portion left over to enjoy the next day.

Do note, if you are new to cooking, that you take care not to undercook your meal, if you scale up your recipe. It is for example easy to overcrowd a pan when frying meats and vegetables.

Maybe I can help with scaling up the recipe you had in mind. What do you want to serve your guests?
Do different cultures have different ways of figuring meal quantities? If so, list and elaborate.
Yes, different cultures have different ways of figuring meal quantities. Here are a few examples:

In some Asian cultures, the portion sizes are smaller, but they tend to have more courses during a meal. The idea is to have a variety of dishes and flavors to enjoy throughout the meal.

In some Mediterranean cultures, there is a tradition of having a large, communal platter of food for everyone to share, rather than individual portions.

In many Western cultures, portion sizes tend to be larger and more individualized, with each person having their own plate of food.

In some African cultures, meals are often cooked in large quantities and are meant to be shared with the whole community, rather than being portioned out individually.

In some Latin American cultures, meals tend to be larger, with each person having their own plate of food, but there is also a focus on communal dining, with everyone eating together at a large table.

These are just a few examples of the different ways that different cultures figure meal quantities, but it's worth noting that there is also a lot of variation within each culture. Factors like family size, socioeconomic status, and individual preferences can all play a role in determining portion sizes and meal quantities.