Here are the answers filled in for your exercise based on the provided contexts:
1.- There are a lot of schools in small towns, but there are very few in big cities.
2.- This beach used to have very little sand, but they brought sand on a lot of trucks.
3.- How much time do you spend doing your exercises? I don't spend a lot of time. I'm a bit lazy.
4.- This library is not very important. There aren't many books, but at my university library, there are a lot of.
5.- There's a lot of wine in the green bottle. In the brown bottle, there is very little.
6.- How many rings does she wear? She's got a lot of rings. Today she is wearing very few.
7.- I've got a lot of free time because I am on holiday, but I usually have little when I have to work.
8.- How much food is there in the fridge? There's a lot of food in the fridge. I did the shopping yesterday.
9.- How many stops are there between these two stations? There aren't many stops between these two stations.
10.- I haven't got much money. I have spent a lot of money this weekend.
11.- The AVE train has got many carriages. Old trains used to have very few.
12.- There're a lot of photos in the album. In my wallet, I carry very few.
13.- How much sugar have you put in my coffee? You like sweet things, so I have put a lot of sugar.
14.- There aren't many cars on the road at weekends. On weekdays, there are a lot of.
15.- If you do your homework today, you'll have very little to do tomorrow.
Feel free to ask if you need any more help!