Quite a few factors involved in max weld pass size....
Since lap joint, assume a fillet weld.
What process is being used? SMAW, GTAW, FCAW?
Is it shop or field welded?
What is the position? Flat, horizontal, overhead, vertical?
What code is involved? AWS? ASME? other?
Generally, 1/4 per pass is common, but for a fillet weld, that might require 3 passes to get proper profile.
1/2 thick plate lap joint needs (by general rules) a 1/2 fillet ON EACH SIDE! to develop full strength.
The first 1/4 inch fillet "might" be made in one pass. To fill the rest of the fillet will require at least two more passes each side at a minimum to make the thickness - but the fillet weld is getting "wider" across the hypotenuse with each added layer => which would require 4 or more passes on each side to avoid an excessively wide bead; so you should expect MORE than 8 passes to complete the joint just a little above the minimum.
This welding will be under AWS D1.1 and design is for IBC 15 in seismic SDS D structure. The plate is 5/8" and per IBC, max fillet size for single pass is 5/16" and can have periodic special inspection. Anything over that requires multi-pass with continuous special inspection. This design needs to have one element designed for yield and all other components of the load path to be at 1.5Sy. My splice plate is the controlling element and the weld needs to be 1.5 times that.
I have ended up using two splice plates to double the length of the returns to get the weld capacity up without the need to use over 5/16" fillet.
