That is a hard question to answer. I do not know how different they are. Of course you can put 5 totally different Markers on the same entry if you like, but it will still be ONE entry. If the Markers are substantially different, it is best to make them separate entries.

But I don't know how different they actually are. That is a determination that YOU the designer must make. Here is the actual rule as is stands in the Official Rules:
11) Individuals may enter as many concept graphics as they want as long as there are substantial differences between each of their entries. This means that color changes are not enough for an additional entry. If you have different color schemes feel free to add small thumbnail pictures TO your submitted graphic showing various color/paint schemes of the same Marker design, but do NOT send as separate entries.
So, as you can see Tom wanted only ADDITIONAL entries if the changes were greater than minor changes (such as a different tank, color scheme, etc.). For a new entry they should be a totally different Marker, not variants of the same one.

I hope this helps, but I think this one can only really be answered by you, since you know how large or small the actual changes are... not me.