Note:Although the definition states that you need "at least 10 subscribers each," the achievement is only earned once each Sporecast has at least 11 subscribers.
Once the achievement is gained, it will not be revoked if the sporecasts drop below 10 subscribers or are deleted.
This is a pretty difficult achievement to earn. If you know people in real life who have Spore accounts, you can make an "I scratch your back, you scratch mine" deal where you subscribe to all of their Sporecasts and vice versa. This will give you a head start, but unless you know 10 people, it won't get you the achievement. Your best bet is to follow two ways to earning it, good creations and advertisement. A Sporecast with bad creations will not attract any subscribers. Neither will a Sporecast that no one has heard of. Try advertising your Sporecasts on the comments section of featured creations. You can also place a comment on some of your buddies creations asking them if you can make an "I scratch your back..." deal with them. Another tip is to create a Sporecast of things that you know that many people will be searching for (i.e. Star Wars, Featured Creations, etc.)
Another newly available idea is to take your EA account and create all 5 possible Spore accounts. Then, select 5 sporecasts created by your main account. This will (supposedly) give you a 4 person head start, in addition to your buddies.
You could also try creating an entirely new EA account and subscribe to 5 by your main account, giving you 5 people in addition to your other 4 accounts and your buddies (assuming you have gotten these.)
The game directly gives the achievement without a pop-up window informing you of gaining the acheivement. All server-based achievements do this.