What type of visa do I need to stay in Thailand for 5 to 6 months on holiday?

That is best done with regular tourist visas. Get one upfront, extend, plan a trip to a nearby country after 85ish days, get a new tourist visa there in addition to the sightseeing activities, and if you stick with 5 months you're then set, otherwise you can extend and reach 6.

As an alternative, you could just hop across a border when the first 90 days run out, get 30 days coming back, and extend those. That'll get you to 5 months as well, but going beyond requires a second trip to a border.

Your local RTE website..

METV up to 9 months check your local RTE website as some countries have different requirements

Multi entry tourist visa you can get almost 9 months

Tourist visa multiple entree

Please check the embassy site in your home country first.

Tourist Visa (60 days)

Extension (30 days)

Land Crossing Border bounce (30 days)

Extension (30 days)

Land Crossing Border Bounce (30 days)