There are practical space limitations on having enough room to physically make a weld. Leading engineering consulting firms commonly specify a number such as ~ 3 inches minimum. The thumb rule is that the minimum distance between adjacent butt welds is 1D. If not, it is never closer than 1-1/2". This is supposedly to prevent the overlap of HAZs. Minimum spacing of circumferential welds between centerlines shall not be less than 4 times the pipe wall thickness or 25 mm whichever is greater.
The Australian Standard, AS 4458 specifies the following: "The distance between the edge of two circumferential butt welds shall be not less than four times the pipe wall thickness or 30 mm, whichever is the greater, unless the first weld is post weld heat treated (PWHT) before the second weld."
If the fittings are 90/45° elbows, sweeps, Tee fittings, bell or concentric reducers that would cause turbulence, but not so much so in straight couplings and unions. A general rule of thumb is 5-10 pipe diameters between any fittings that may induce turbulence. Of course a good piping designer will take care to control the velocity, and turbulence in his pipe designs.
If you have bolted flanges, make sure that you have enough distance between the pipe flanges to enable you to swing a socket wrench or pipe wretch.