This is kind of a difficult question. Thai proper has these basic script (and likely I am forgetting one or two), in order of time
1. Ramkhamhaeng script(still debateable on authenticity)
2. Sukhothai script
3. King Lu(?) Thai script
4. King Narai reform script
4a . Compressed Thai script
4b . Modern Thai script
5. King Monkut reform script (didn't catch on)
6. And there was another proposed reform that didn't catch on either..will have to look it up tonight.
And somewhere in this list, the Khom script should go. That's Thai written with the Khmer (Aksar-Mul type) script, and a few extra letters and marks for tones.
Tai languages (Lao, Dham, Tai-Deng, Tai-Dam, Lanna, Khuen, Lue, Shan, Tai-Neua, Tai-Lai-Chau, Aiton, Ahom, Khamti, White Tai, SonLa Tai, Pai-Yi, Tai Na, Sipsongpanna, etc) all have their own apparently-Mon derived script....with those in the Yunnan region being kind of diamond-shaped due to their being written with a brush on paper/bamboo rather than on a palm-leaf with a stylus....suffice it to say there are A LOT of different Thai/Tai scripts.