Bettas need at least 2.5 gallons...but the bigger the better, really! Be sure to provide him/her with a hiding spot and some plants so they feel secure, especially in larger tanks! Silk and live plants are ideal. You can get plastic as long as it isn't sharp enough to tear a betta's fins. they're very delicate, so you'll have to be careful about decor.
Heaters are a must. Bettas are tropical fish, so they need a temperature of at least 76 degrees, with 78-80 being the most optimal.
Filters aren't necessary, but cycling your tank will be a big help. If you don't get a filter, you just need to be prepared to do 100% water changes. How often depends on what size tank. (A 2 gallon non-filtered tank will need to be cleaned out more often than a 10 gallon tank, for example).
If you do choose a filter, be sure to pick one where the flow isn't very powerful. Well, you could always build a baffle if it is...
Whether you want a single betta or more than one depends on you. The smallest tank you can split imo is 5 gallons, so each betta has 2.5 gallons.
Currently, I have a 10 gallon split into 3, so each betta has something like 3.3 gallons.
If you want to split a tank between a male and a sorority on one side, get something that allows the female sorority to have at least 10 gallons. So, a 12 gallon + tank. Females can be kept together in groups of four or more. They do better in odd-numbered groups. Be sure to include a lot of plants and hiding spots for them!