It's important to remember that most, if not all, of your trekking income will arise during the late spring, summer and early autumn. Many people do not enjoy going for long rides during the cold winter months, particularly when it is wet, windy or snowy.
The trekking 'season' is a bit longer in the south and west of the UK than in the north and Scotland and you'll need to make a realistic estimate of the number of weeks in the year when you are likely to be busy. Realistically this is likely to amount to between 100 and 120 trekking days per horse per year.
You may be able to offer other services during the off-season winter months which will tide you over, for example, riding lessons or livery services. Bear in mind that you will have to feed and care for your horses even if they are not earning any income. Similarly, you may have to pay staff wages throughout the year, not only because you need staff to look after the animals but also because you won't want to risk losing well qualified and experienced instructors and ride leaders.