This post is for Tui:
1. As long as someone does not practice all the asanas of the primary series it is recommended to practice 5 surya namaskara A and 5 surya namaskara B. The reason: it is important to get strong.
2. When all the asanas of the primary series are practiced, only 5 surya namaskara A and 3 surya namaskara B are practiced. Often I practice with the CD by Sharath (the grandson of P. Jois). It's exactly how I wrote, he counts me through 5 surya namaskara A and 3 surya namaskara B.
3. Feel free to do what is good for you. Perhaps you feel cold during winter time. In order to warm up, it can make sense to practice more surya namaskaras. After a long break a yogini might be weak and he/she might not be able to do all the 8 suryas, then it is recommended to work on adding again one by one.
4. Sometimes there might be only time for surya namaskaras. Then that's it. Do as much as you like, it's a great warm up and it stretches the body as well.
Thank you for buying your Christmas presents via my blog.