To be fair, they limit it mainly due to the levels of people coming to the country solely to CTB. If they permitted more people, I'm fairly sure governments would take note of that and relations could deteriorate. I doubt any countries like the UK would be pleased if they saw hundreds of their citizens undergoing assisted suicide in a whole other country. In fact, I'm fairly sure there are companies allowing individuals from other countries to go through the process, I just think you have to get Swiss citizenship first before being eligible, so you'll probably have to naturalise first.
I am unaware of any quota or limitation on how many foreigners are assisted. Based on what I can find, at least 60% of Dignitas (probably even more for Pegasos) are non-residents, Germany being the main country. Dignitas, from its inception (2000), made their doors open for foreigners with the same criteria as residents, every person is evaluated on their merits. Exit does provide more, and they do NOT accept foreigners. Taking that into account, I'm guessing about 20% of Swiss assisted suicides are foreign residents. What may limit the numbers, perhaps, is the logistics and cost of getting through the application process and of making the often arduous trip to Switzerland, especially if you're physically compromised. About the same number of DIY (unassisted) suicides in Switzerland each year. Similar reasons/complaints/diseases for both, the exception being "depression": 55% for DIY versus 8% for assisted. That reflects the difficult hurdle of the legal requirement to be of "sound mind." And finally, Dignitas spent 154,000 Swiss francs last to reduce their fees or make their services free for those who could not afford it. Which probably covered 10 to 20 people. This is somewhat covered by 124,000 Swiss francs in donations.
What IS impossible is to just show up at their door expecting to get an immediate assisted suicide. You do have to do the leg work.