Question about Military Starbase

They can be anywhere, subject to the limit of 4 per sector. Note that resource mining starbases don't count towards that total.

But if a planet is within the starbase (SB) Area of Influence (AOI) then any ships in orbit about the planet will receive the SB bonus which is a benefit if that planet is attacked but the planet receives no direct benefit from the SB.

The question is more what you're trying to accomplish with the SB. I knew someone that essentially used military SB's as a "fence" around his area with fleets of 1/1 fighters that became dreadnoughts in the shadow of his fully populated military SB's but that took a long time to setup and was simply that person's preferred style of play.

Often starbases but particularly resource mining SB's are favorite targets of the AI so I could see putting one off to the side to draw away the enemy to their death by the SB enhanced ships underneath.

So I guess the best answer is that it depends on what you're trying to accomplish.

Another use of the mil bases is the catapult effect.  That is, by picking a spot to overlap a lot of bases with the speed boosters, one can have ships and fleets go a very long way on that one turn where they get all the stacked speed bonuses.  On smaller galaxies, one can essentially conquer the map on one turn.  On larger maps, one can conquer a race or two in one turn.  To get the max advantage, one would choose the desired spot and then locate the bases, taking into account the 4-per sector limit but maximizing the overlaps.

And to the contrary, speed-nerfing modules coupled with the Isolator-SuperAbility will freeze enemy ships when they try to approach your own AoI.