what is the rule about sideboard size? what amount? up to or must be?

Up to 10 cards — the sideboard is a maximum, not a fixed count.

Rule 601.1.c.1 (Tournament Rules): "A player's sideboard can include 10 or fewer cards" in constructed events. Sideboarding is done 1-for-1 with Main Deck cards between games (403.4), your Main Deck stays exactly 40 (402.1), and copy limits apply across the Main Deck + sideboard combined (403.3). Note: older FAQs reference an 8-card limit from an earlier rules version — the current rules allow up to 10. In limited formats (sealed/draft), the sideboard is instead everything in your card pool not used in your starting configuration (602.4.a.7).