Keep it to a minimum. Firstly, it's really hot in Thailand, even at night, so the hoodies and sweatshirts with long sleeves in which you will fly are more than enough. The same goes for jeans and trousers. You are unlikely to wear them, so do not clog the suitcase.
In general, in terms of clothing, the layout is as follows: 2 pairs of shorts, 4 T-shirts + 2 swimsuits or bathing trunks per person, well, underwear at your discretion is quite enough for a vacation of 2-3 weeks. Well, it's clear that the girls will take with them a couple more dresses or sundresses to go to a restaurant in the evening.
Secondly, it will be cheaper to buy all summer clothes in Thailand than at home. And you will probably buy it here, so take care of the space in your suitcase. The exception is branded items in the shopping center. For example, a swimsuit on the market costs 300-350 baht, a Roxy swimsuit costs 3000 baht (about 5800 rubles). Therefore, such purchases are cheaper to make in Russia.