Modern enema bags, from pharmacies, are typically 1.5 quarts to 2 quarts. That is a safe amount for most adults to take. I suggest you keep to that volume until you know you can safely handle more.
Back in the 1940s & 1950s enema bags typically held 3 quarts of water. Again most adults can handle 3 quarts. But I wouldn't try it until you're sure you can take it. There are safety reasons why modern enema bags are smaller.
How much volume you can take will depend on how big of a person you are (a large man can take more than a petite woman.)
It will also depend on your experience. I was taking 3 quart enemas for decades before I could take 4 quarts. It's kind of like a marathon: you can't just put on sneakers and run 26 miles the first day. You have to work up too it.
In general, as the water flows in you will start to feel tight and uncomfortable. Stop then. Especially if it becomes painful! You don't want to hurt yourself. Most people discover they can take more volume after they've flushed out a few times and sort of stretched their inner muscles a bit.
I also suggest using water that is comfortably warmer than body temperature. Allow the water to squirt against the inside of your wrist. If it is comfortably warm it is OK for a comfortable enema. Warm water will also help you insides to stretch. (Your colon is basically made of protein and protein becomes stretchy when it is warmed. This varies if you are an overweight person: fat does not stretch like protein.)
If you use soap, use something mild like Ivory Soap. Or use a few tablespoons of salt per quart. As a beginner, don't bother with elaborate enema solutions. If you wish to try coffee enemas, be sure the coffee has cooled to about body temperature! Hot coffee, right out of a coffee maker, is way too hot to use safely! (And it will be agonizingly painful!)
Some people like to hold and retain their enemas. You might find that a kitchen timer is handy to measure your retention time.
Many people like to take a series of enemas spaced a reasonable time apart. (Again, a kitchen timer might be helpful.) This will really clean you out. It may also leave you tired. So plan on an afternoon of several enemas over several hours followed by nap time!
Guzzleguzzle Kid