Once you've chosen a perception task, it's time to make stimuli for it.
How many stimuli do I need?
The answer to this question isn't simple. You'll need to strike a balance between getting a sufficient amount of data and how long you can reasonably expect people to sit and do your experiment. In our lab, we generally have to recruit participants with extra credit, the promise of snacks, and desperate pleas, so any experiment over an hour or an hour and 15 minutes is unlikely to have many people sign up. If you can pay people they'll be more willing to do a longer experiment, but that means more money you'll have to shell out for each person. Since your experiment is likely to be made up of two or more tasks, such as both discrimination and lexical decision plus a background questionnaire, each task in itself shouldn't be longer than about 25 minutes, if possible. Shorter tasks will also prevent participants' attention from wandering too much, which means more reliable data. A 20-minute AXB or oddity task is already very boring even with a break, and with difficult contrasts it can also be mentally taxing and demoralizing. I know some psychology experiments have participants doing one repetitive task for an hour (how?!), but if you don't want participants to constantly time out on trials because they are falling asleep or trying to surreptitiously check their phones, keep it shorter.