Will I have to give lots of sperm when I get a gradient sperm washing, filtration and improvement?

No. If you are expecting to get pregnant via insemination, a single ejaculate should provide enough sperm to achieve pregnancy. If the total number of motile sperm is low after the wash, then what you may need is ivf. Sometimes we will do "back-to-back" inseminations, but there is no data that this significantly improves the odds of pregnancy.
No. We ask for a single sperm ejaculate for gradient sperm washing. The exception may be for ivf when the man has a very low number of sperm after processing and we have to inseminate the eggs that day. If the sa test suggests low counts we have suggestions before iui. If the wash yields low numbers for IUI then rethink IUI for the future. Space does not allow full explanation, see your doc.