No you are not but you are limited on file size. And Custom critters (contacts/AVs-EBs/Groups) take big chunks in file size. If you just want to make a hack 'n' slash map you are free to make as much as you want but if you want to make a story you will be probably limited to 3 or 4 missions on tops depending on custom group you made.
Well, I have an arc with 2 minions, 2 lieuts, 1 boss, 2 EBs and an AV, and I'm at about 80% with 5 missions.
10 or 11 is about the maximum number of custom mobs you can fit in an arc.
I really should do something about this signature.
Best thing to do with your custom groups is make everything you want to use, and chuck em into one large pool. Using that huge pool of bods you can then create little groups which will let you see how much space you use up.
For instance I have an arc that I'm working on which needed some varied lab assistant style creatures to attack. I created every type that I needed and placed the whole lot into a group called "meds" using that group I was able to create little groups "Students" "Teachers" "The Mistress" for every mob type that I needed. If after that I found that "The Mistress" knocked me over my size limit I can cut down one of the groups till she gets used, or find some story based way of not using her without affecting the rest of the mission.
Another thing I'd recommend is spicing up your groups with a couple of standard mob types. It's best to choose groups that cover the whole range of levels like Council, and 5th column...but if you want to use, say: Carnies (and I'm all for that) which start at level 30 you'll be autosidekicking anyone less than 30 and thereby making their lives much tougher. I throw in groups from across the myriad types that cover all the level ranges when I use them. 
I don't wanna save my soul now,
I just wanna lose control,
And even if it takes a lifetime, to learn:
I'll learn!
Generally, if you must have a custom faction, it can be a good idea to make one of each rank - minion, lieut, boss - and finish the rest of the arc, then when you're done and have space left over, feel free to throw in some alternates for variety.
ok i got 2 of ever type atm, and still have 20% left for a 3 arc atm
Got three minions, three lieutenants, two bosses, one EB, one AV, and two allies... that's twelve, and in a four mission storyarc, I'm at like 99.59%. :3
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Arc: 60092 - Supa Rumble in the Park
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i wanted to make my own group to beat up, am i limited to how many different minon, liues, boss and so on i can creat for them to show up in the map?