Which race's turrets are you trying to place? Turrets are limited by the number of HQ's you have built. I think the IG, Space Marines, and Chaos are able to place 8 turrets per HQ. Not sure about the Eldar. The Ork turrets (WAAAGH!! Banners) are directly tied to their economic system and may be built without limitation so far as I understand. The Necron are limited to 4 turrets per Monolith structure. And the Tau's only turret comes from upgraded listening posts to balance the insane rage of fire that their units can achieve with upgrades, in addition to being the strongest "shooty" race in the game (weakest at melee).

The numbers are limited for balance reasons. The Necron turrets, for example, are the best turrets in the game. Unlike other turrets, the Necron turrets when fully upgraded are more powerful and more effective against all units, not a specialization of either anti-infantry or ant-vehicle. That is why they are limited to 4. Add that to the fact that you can now place a turret on any visible portion of the map that isn't obstructed; you no longer need to place them within the control area of your buildings, and turret's value becomes a whole lot more than just defensive.

This also frees up space at your main base and allows builders to join in a more offensive role at forward positions. The Imperial Guard are especially adept at placing and maintaining offensive turrets with the most durable builder unit in the game (wears commander equivalent armor + higher HPs).

And Ninja is absolutely correct. Map control is vital to a strong economy, which tends to preclude heavy base defenses. Once the game progress in to T3 and T4 stages, turrets get owned pretty quickly anyway, especially if you have lots of them all clumped together. Think the Force Commander's Orbital Bombardment ability or Land Raiders that take out a turret in two shots... or Eldar Fire Prism Grav Tanks that do high area effect damage, leveling groups of turrets at a time if they are bunched up, etc.

Turrets definitely play a role, but one shouldn't go bonkers on them until one's economy can support it, usually after everything else is built and upgraded. By that time, they are usually only good for slowing down the enemy's incursion into your more important base structures, giving you time to respond.

I find that I almost never build turrets during the early game unless I am getting rushed and can’t stop the attack (maybe I am busy rushing his ally’s base, or have spread my SP capturing teams too thinly). At that point, I build a turret to cover parts of my base that I expect him to move toward once he’s through with his current target, which buys time to respond.

The other thing to remember is not to panic when you get rushed early in the game. T1 infantry take forever to do any significant damage to buildings. Your base will take a beating for quite some time, giving you opportunity to finish capturing strat.points before responding so that your economy doesn't take a hit as well as your base. Focusing on expansion and economic development (map control) early wins the day, not turrets. The only exception to the rule is the Necron who build their economy at their base in the form of power generators. However, even they need map control in order to field their armies. Their population and vehicle cap is tied directly to the number of completed listening posts (obelisks) they have built over strat.points and/or relics.