Number of hands
As Sonny suggested, for the Basic Strategy player, spreading the same amount of money across several hands will cause you to lose less in the long run.
Here are some AP considerations:
1) During high counts, some counters will spread to multiple hands to get more money out there - either because they've hit the table max bet, or for cover (e.g. jumping from one hand at $50 to two hands at $100 will look less suspicious than continuing with one hand and betting $150-200).
2) Spreading to multiple hands burns through cards faster. During high counts this is to the counter's disadvantage, but is often necessary for the reasons outlined in (1). During negative counts, it's better to burn through the cards faster, and under certain specific conditions (a small casino with one BJ game open, and the counter is head's up with the dealer), it may be beneficial for the counter to play multiple hands with the smallest bet possible.
3) If a shuffle tracker is expecting an ace or a clump of aces to be dealt, they are likely to spread to multiple hands, increasing the likelyhood that one of their hands receives the expected ace(s).