HMRC will calculate your average profits based on any tax returns you’ve submitted over the last three tax years.
This means that if the average profit over the three year period is under £50,000, you’ll be eligible for the scheme. If the average profit over the three year period is over £50,000, you will be ineligible for the scheme.
If eligible, you’ll get 1 payment to cover the whole length of the scheme, which will be paid in June.
More here: https://www.citizensadvice.org.uk/work/coronavirus-if-youre-self-employed/