Creatures with Defender cannot block more than one Creature when assigning blockers, unless the blocking Creature specifically says it can. The basic rule is this: blockers can only be assigned to a single attacker. You may assign multiple blockers to a single attacker. Whether or not the Creature blocking has Defender does not affect how many Creatures it may block.

What makes Defender special?

Defender is a Static Ability which lives on, or can be granted to, Creatures. A Creature with Defender cannot Attack.

Back when Magic: The Gathering first started, the Creature type "Wall" automatically meant the Creature couldn't attack, but all those cards have been errata'd to have Defender.

See? In its early printing, Wall of Swords was a Wall card (with the old language "Summon Wall"), but when it was reprinted later, it got Defender. So, if you encounter an old Wall card, it can't attack.

(I always liked Wall of Swords.)

Very often the old Wall cards included rules text to explain that the Creature can't attack.

There is one old Wall which allows you to Attack with it, Mobile Fort. You can pay to turn this 0/6 with Defender into a 3/5 without it. Note the modern language erratas this Creature to have Defender and to lose Defender when you activate the ability.

There is exactly one Creature with Defender which can Block any number of Attacking Creatures: Wall of Glare. It was only ever printed in Urza's Destiny. Note it says "Wall of Glare may block any number of Creatures each Combat" (emphasis mine), but the new Oracle text changes it to the modern language, which uses "can" instead.

Additionally, there are effects which allow a Creature to Block more than one Attacking Creature, such as Entangler.

If you Enchant a Creature with Defender with Entangler, now your Creature with Defender can Block any number of Attacking Creatures.

Why bother?

Creatures with Defender, even though they don't change your Blocking math, can have a lot of value.

• They're very often aggressively priced (a 0/3 for 1 mana can Block a lot of early Attackers and survive)
• They often have powerful abilities (consider something like Wall of Reverence, which is a 1/6 Flying Defender Creature for which gains you life equal to the Power of target Creature you control. At the very least, it's a free life each turn; if you have something bigger, it's even more)
• There is often synergy with Defender

With a group of Creatures with Defender on the Battlefield, you can activate this ability and do a decent amount of damage.

Or you could take on a milling strategy, attempting to run your opponent out of cards.

In the Commander world, few Legendary Creatures show the reason for Defender more than Arcades, the Strategist.

In addition to drawing a Card every time you play a Creature with Defender, your Creatures do damage according to their Toughness (Creatures with Defender often have high Toughness) and they can Attack as if they didn't have Defender.

Final Answer

Creatures with Defender can only Block one Attacking Creature in most cases, just like any other Blocker. A card with Defender is unable to Attack, but the keyword "Defender" does not change how the Creature may block. There are ways to allow Creatures with Defender to Block Multiple Creatures, but without assistance, most Creatures with Defender can only Block one Attacking Creature.