There are two separate questions regarding coalitions in Thailand. The first question is with regards to a coalition to select the Prime Minister. Given that this includes 250 junta-appointed members of the Senate in addition to the 500 members of the lower house, Prayuth’s party needs only secure an additional 126 seats in the house. Polls have placed his party as gaining anywhere from 5% to 25% of votes (25-125 seats). Many commentators and the most recent Chulalongkorn poll is placing that conservatively in the middle at 70 seats.
Opposition parties, in contrast, must club together 376 seats of their own, all in the lower house. Best estimates of Pheu Thai are around 200 seats, with the Democrats at half that or around 100. If, and it is a big if, those two parties could agree to enter a coalition together, then they still need an additional 76 seats. Future Forward, which has been estimated to get around 50 seats, would leave only 26 seats. Phumjaithai may be the kingmaker in this case. Originally one of the successor parties to Thaksin’s first electoral vehicle, Thais Love Thais, Bhumjaithai may be able to get 26 votes. In many ways, Bhumjaithai is more of a sure bet than the Democrats, as long as the Democrats have already agreed. If the Democrats refuse to join with Pheu Thai (or vice versa), then they are just as likely to throw their support behind Prayuth.
One question that remains if the pro-Democracy parties will form a super majority coalition. This is a common scenario in times of national emergencies. And the current political climate seems as close to a national emergency as the country has seen for some time. All parties will likely be invited to join the coalition in this case. Prayuth and Suthep’s parties would likely refuse to join, but we could still see over >400 MP government coalition.
A perhaps bigger question is how Prayuth will form a coalition if he wins the premiership? That scenario seems the most likely still, but he will need a majority in the House to pass any kind of legislation. The Democrats said they would not support Prayuth as PM, but might support his policy proposals in the legislature. Regardless, it is likely that legislation will happen on a case-by-case basis.