The seats won in 2017 General Election were
Labour + SNP + LD +Plaid + Greens 314
SF 7 don't vote
Independent 1 (Lady Hermon) from NI will never vote for Corbyn
DUP 10 also will never vote for Corbyn
Take 1 off for the Speaker and another 1 for his deputy. The other deputy will come from Labour
So the Tories have 316 voting MPs. I make that a majority of 4 for May over Corbyn's coalition of chaos excluding DUP MPs. Its tight and they could easily lose votes but this can always be reversed the next day by making it a confidence vote. The Queen's speech is always a confidence vote..
The IRA have had several goes at murdering various DUP MPs so they will never vote for IRA apologists like Corbyn and McDonnell.
It would be difficult for May but not impossible. In a crunch she does not need the DUP but she has made her bed and now she must lie in it.