How many magazines is enough? That is a question from a lot of beginners! The answer is not so simple. First, you should know in most cases for your AEG that you have options on the types of magazines available.
In airsoft you have 2 basic types of magazines: High Capacity and Mid
Capacity. The main difference between these two types is the method in
which they are loaded and feed.
High Cap Magazines
A high capacity is essentially a hollow magazine body that can hold a lot of ammo. There is an internal gearing mechanism with a wheel that you have to manually turn to tension a spring inside the magazine. This spring interacts with the gears inside and loads BBs from the main compartment up a chute and into your gun. The magazine can be wound a certain number of times before you cannot wind it further and allows the gear to slip to prevent damage inside to the components or spring. Once all the spring tension is used up you will have to wind the magazine again. A typical high cap magazine can go through three or more winding cycles to completely deplete all the ammo in the mag. This style of magazine does not require any tools to operate.
Mid Cap Magazines
The mid capacity magazine is more traditional with a simple spring and a plunger/follower. You would load BBs into the top of the magazine using a loading tool. The loading tool is a device that manually transfers ammunition from the tool into the magazine with the press of a plunger. You will transfer ammo from the tool into the magazine until you feel you can no longer depress the plunger. That means the spring is fully compressed and the magazine is full. It requires no further loading or winding after it is loaded and is ready to go the moment you insert it into the gun. It will feed BBs to the rifle until it is completely out. These generally feed very reliably and take longer to wear out. The BBs also don't move around inside the magazine body making loud noises.
Okay, now that you know what kind of mags exist for AEGs, it's time to discuss logistics. If you’ve just picked up your gun brand new, chances are you will have just the high cap it comes with. Very few guns might come with a mid capacity magazine as standard instead. A few brands may include a second magazine for added value which is nice.
To get an idea of how many magazines you should have, you should ask yourself how often do you think you’ll be shooting in a day. This will depend on the size of the field you're playing at, what kind of role you might play, and if there are large teams and respawn rules. If you're at a particularly large and busy field, you'll probably have more things to shoot at and therefor run through more ammo. If you have rented and played at a field before you might even have an idea already of how you play and how much ammo you would expend in a round.
With a high cap, you could carry some bottled BBs and reload out in the field and get by with one magazine. Though if you are in a pinch this might be slow. Though it is not ideal, you could do it for now. If you fancy yourself a trigger happy shooter you may need more magazines for sure. And also consider the amount of players you might see in the field. If your local field has only small groups, you won’t need that many mags and ammo. If your field is particularly a busy one or you are going to an event, again, carry more magazines. Generally if I see you starting out with an M4, I’d carry at least 2 high cap magazines (= 600 rounds in total). Even more than that is ideal for the larger venues. Some guns like the AK hold a lot of ammo and it would be more than sufficient with just one.
I’m a mid cap user myself so I tend to carry 2 or 3 times as many magazines to match what I could carry with just 4 high caps. The mid cap magazines for the M4 that I like to carry hold only 130 rounds. That means I would have to carry 8 magazines (1040 rounds in total) or I could carry just 4 High cap magazines (1200 rounds) and still have more ammo. The reason people are picky about the types of magazines they use are because of play style.
Two things to note: Magazines can wear out or break over time. Lubricate them with silicone oil to keep them working longer. If your gun is not feeding the magazine is probably the first thing I would check.
The other thing worth mentioning is that you should have extra batteries with you to match up with all the shooting you'll be doing.
Alright so that should give you some math homework to think about! See you out on the battlefield!