The number of cells is probably the least of your worries. What sort of regulator did you convert it over to?
If you're still running the stock charging system, a lithium battery is a bad idea. Modern battery tech does not play well with old school charging systems. Even with a modern regulator, some battery companies claim their lithium batteries should not be used on old school bikes because of the variation in the charging voltage, even if it is under the maximum allowed. And many modern aftermarket regulators specifically say they aren't to be used with lithium batteries.
At the very least you need a MOSFET regulator. The re-start antigravity batteries apparently have a BMS built in but even that isn't a fail safe. I don't know which batteries they have are 4 cell and which are 8 cell but if you're looking at the micro-start stuff with no BMS you're rolling the dice. You can kill it by running it down too far, or by putting too many volts into it. For a lithium battery, the max charging voltage is somewhere around 14.9. Does your regulator always keep the voltage below that at all rpm? Are you sure? Is there any stray drain on your bike that will run the battery down as it sits? Get it wrong and the battery is shot. No recharging. It's done. Buy a new one.
From the anti-gravity site:
Hi-Power lightweight ATZ7-RS lithium powersports battery with RE-START Technology (built-in jump-starting). Replace YTZ7S, YTZ5, more. 150 CA.
The good news is, Anti-Gravity uses lithium iron
which isn't as prone to flaming up as lithium ion batteries are. So if you overhcharge one, you'll likely only get some impressive arcing and really nasty smoke as it melts. It probably
won't actually start burning.
IMHO a battery with no built in BMS doesn't belong in a bike. I say that as someone who's melted one down that HAD a BMS, but when you completely lose the regulator things get crazy. This took over 60 volts to melt down but a '91 Ducati has no problems doing that when things fail. With no BMS, 16 volts could eventually lead to this.
I'm running a lithium battery in an older bike that probably shouldn't be using one. I'm also running a MOSFET regulator and a volt meter. IMHO you need both of those, along with a battery that has a built in BMS to run a lithium battry in on old bike. And even then it's not "best practice".
If you must use a lithium battery;
Get a modern regulator
Install a volt meter
Don't use a battery with no built in BMS
Don't bitch when it still goes wrong.
Rough guess is you'll have about $350 into putting the first lithium battery in there. Try to cut corners and you'll have about $500 into it with the second one. Or you'll save money and find room for an AGM the second time around. Ya might want to find room for an AGM now and avoid all the costs/time of the lithium...
And what makes me think any of this can happen??? (NSFW language)
To answer your original question (sort of), buy the smallest Re-start version they have. It should turn it over fine. If it doesn't, cowboy up and use the kickstart.