Fear not. 20 is fine. My husband and I BBQ'd for 70 people a couple of years ago (to celebrate our little garden being done) and it was great fun. Prep is the key!
Also, congratulations on the garden ☺️
Plastic baskets that hold paper plates work really well. Get some paper napkins and then stack them the day before. Similarly, plastic cutlery is a no brainer. Also, have bins / bin liners to hand. Put all the plates and cutlery somewhere together.
Nibbles set out in bowls around the place - crisps, nuts, whatever - helps to stagger people's hunger and let's face it, everyone loves to snack, especially if they're drinking.
Food-wise... Have a table / area with condiments (tomato sauce, BBQ sauce, mustard, mayo, gherkins... Whatever else is in a bottle / jar and might go on hot dogs / burgers), burger buns, hot dog rolls, cheese squares, lettuce, onion etc largely set up in advance.
Somewhere else (but not too far away) have your sides. Ones I find that work well (because you can make them ahead of time and they're inexpensive):
- homemade coleslaw (a cabbage, carrot, red onion - slice it all up in advance then refrigerate - add the mayo and mustard / Worcestershire sauce / whatever just before you serve it)
- pesto pasta with cherry tomatoes and pine nuts
- rocket and parmesan salad (grated parmesan onto rocket, sprinkle on some salt, drizzle olive oil over just before serving)
- homemade wedges with either mixed green herbs, garlic granules, salt and pepper or cajun mix
I'm not sure if you're also putting on drinks for people but, if you are, keep it simple. We've found boxes of Corona, bottles of Prosecco, bottles of French red all tend to go down before other stuff. You could buy in a crate of coke cans and a crate of lemonade cans for the non-drinkers. Depending what else you're offering, it might be a good idea to have ice in, too.
When it comes to cooking, you'll be fine getting your sausages and burgers on the BBQ - do a few of each at a time and people will come in drips and drabs because they'll have the other "stations" to go to. Have someone on hand to run between the BBQ and the kitchen with the raw meat and clean plates to put the cooked stuff on (if it ever makes it that far). Alternatively, find a keen BBQ-er and give them the reins whilst you sort the rest of the stuff. Very sexist to say it (I know but stereotypes come from somewhere), men tend to love to BBQ together and can stand, chat and turn meat with a surprising amount of competency... The fact it's a charcoal BBQ will only serve to ignite their manly senses! You might find they're asking to get involved... We usually end up with a man huddle around the BBQ!
The main thing is to enjoy it. People will be having fun chatting, admiring your garden, enjoying a drink and eating whenever food is served. BBQs are relaxed events so it's hard to get it wrong. Just get those charcoals going early enough that they're the right temperature at the right time. If you're really concerned, you could buy a couple of disposable BBQs to chuck on top of your BBQ once the coals are done (so you basically use your BBQ to rest the disposable ones on).
Hope that helps ☺️