I haven't sewn roll bar covers yet so I can't tell you how many yards you need but if you find a shop that's willing to do the work for you they'll be able to estimate the yardage necessary.
What may be hard is locating the twill fabric, so this may help: https://www.haartz.com/index.php?q=exteriors/twillfast
. The factory soft top twill fabric is Haartz Twillfast RPC which can be found on that page. Haartz doesn't sell small quantities to end users, so you'll probably have to call around to automotive upholstery and convertible top shops to see if any of them have the fabric or can get it for you and if you're lucky you'll find a shop that's both willing to sew them for you and can source the fabric. A few years ago I checked online and found a few places that would sell it in small amounts and it's about $90 per yard so sourcing new fabric and getting a shop to do the sewing for you will probably be an expensive project.
Another source for fabric is Craigslist - I've bought a number of used twill soft tops I found on Craigslist, most of them being sold because of bad zippers, bad windows or some other flaw people didn't know how to deal with but the tops were a great source of reusable fabric and the reclaimed fabric from each top (2-3 yards depending on whether the top was from a 2dr or 4dr JKU) was way less expensive than buying it new. Another way I got some - when my local dealer moved to a new building a few miles away from their old one they had a bunch of soft tops in their storage room that were the result of warranty replacements and they didn't want to move them so they gave them to me and I'm still using the fabric from that haul for new projects
I've sewn quite a few things with twill from used soft tops, including the roll-up soft sides (shown rolled up in this photo) for my homemade LJ Safari Cab hardtop and the cargo bag hanging on the spare...
Twillfast RPC is a really nice fabric to work with, but may be a challenge for people sewing at home - an inexpensive home sewing machine may struggle to sew through multiple layers, it's pretty thick and the typical mostly plastic home sewing machines around today may not handle it. That wouldn't be a problem for an upholstery shop but I mention it in case someone wants to try sewing with it themselves.