forwarding loss

"the guy started his business on oct/19, it was a startup expense"

Use the IRS for your resource. Get Pub 535 for 2019, Chapters 7 and 8 help you understand start up costs, for what is and is not Loss. Don't confuse Spending (cash flow) and Loss or Expense.

"he signed up with qb on Jan/20."

If you are doing the 2019 taxes and he was in business in 2019, then you use Pub 535 that is labeled for 2019 tax returns, to file the 2019 taxes.

"My question is how much can I forward from the loss which is 16000 to 2020"

Filling in the tax return will help you what is carried forward.

"by the way his net income is 55000"

Is that for 2020? You stated "net" but is this before you have worked on his tax return? An operational Net isn't the end result. You will need to review what they are reporting and how they categorized it.

"the other question what tool can I use is journal entry fine or there’s a better way." Yes, there will be some entries to make, such as the Capitalization and Amortization for the tax year. Yes, that is typically done via journal entry for the Bookkeeping entries from the tax reporting. If this is a QB question, you don't use journal entries for anything where names or products and services are part of the adjustment.