It doesn’t matter whether you organise a one-day event or a festival weekend: your visitors deserve a clean toilet. I know… it’s difficult maintaining the cleanliness of those portaloos on a busy event day. But still, there’s no reason for them to be disgusting.
How many toilets you need at your event site depends on the duration of your event, the nature of your event, the weather conditions, and the audience you’re expecting. There’s a nice calculation in the English Event Safety Guide that gives you an idea of how many toilets you need. The Guide says:
For an event that lasts 6 hours or less you need:
1 toilet per 120 women, 1 toilet per 600 men, and 1 urinal per 175 men
For an event that lasts for more than 6 hours you need:
1 toilet per 100 women, 1 toilet per 500 men, and 1 urinal per 150 men
Position these toilets not where it is convenient for you but where it is convenient for your visitors. Don’t cluster them all together (yes, I've seen that happen as well) but spread them out based on how your crowd moves on site. Make sure you can actually get to them to clean them.
It is doable to have portaloos with lights in them on site. Everyone needs to use the toilet so do more with this area so utilise the space to your advantage. Make it more interesting for your audience and yourself!