Speed is a calculation of weight, combined engine power, and a set value modifier. There is no coded cap (I believe that was confirmed on a dev's stream once), but there will be a natural limit due to a combination of maximum weight and maximum amount of engines with max power that fit within that weight.
It's worth remembering as well that engine weight contributes to the weight which slows you down, so adding more engines won't always give a huge bonus. It really depends on your ship but in many cases, more than 6-8 engines just become a waste of resources and weight that could be better spent on defences.