My reel is rated at 12lb can I go past this? What issues will I run into besides less length fitting on the spool.

Knowing the exact reel would help.

 

For example, my Trion-30 is 'max rated' at 130 yds of 8# mono...last year I put 12# Flouro on it. With the calculator located at https://www.pattayafishing.net/fishing-reel-line-capacity-estimator/ I figured I could put 77 yds of that on the spool. Since the max drag of the Trion is 9lbs, but I set it to around 4lbs, this worked fine as I can't cast a lure 77 yds.

 

Far as I've experienced, you CAN use line heavier than the 'recommended', it's just not that common.

Reels don’t have true line ratings in that you have to use only that line rating. It is only describing a capacity it will hold. 

For example if is states some line 10/200, the reel’s spool will hold approximately 200 yards of line, usually mono by default or otherwise stated. 

This rating doesn’t mean you can’t use 8# or 12# line, but the spool will hold more or less of a given line. 

You can extrapolate or make an educated guess that is will probably hold 12/175 or something like that or 8/225. 

 

Also, just because a spool will hold a certain amount of line, it does not mean the reel will automatically work well with it. It is only telling you a capacity. 

For example, don’t expect a typical fc line that isn’t supple in its nature, say 14# fc line to behave well of a 2500 size spinning reel. Or, if you put 10# braid on a casting reel, you could probably expect to experience “dig in.”  

 

You didn’t specify the reel type. General rule the thinker the line with a spinning reel, the less manageable it might be. Casting reels generally work well with thicker lines and even thinner lines down to say 8#. Lower that that if it isn’t finesse, you might be taking a risk fishing heavy cover with 8# mono. 

Your original question has been answered already, but just to add

19 hours ago, islandbass said:

For example if is states some line 10/200, the reel’s spool will hold approximately 200 yards of line, usually mono by default or otherwise stated. 

 

Approximately is putting it lightly. For example, an abu garcia pro max 30 (spinning reel), says it will hold something like 150 yards of 8lb. I've filled up about 30 of those spools, and breaking 90 yards with a fairly tight wrapping machine is a quite a feat.

Those numbers are just a loose reference for line capacity.  No harm will come to your reel from using any line you wish.  

 

 

 

 

I'm talking about these reels, Not sure which one I will be getting but i want to use 15LB mono.

 

https://www.ebay.com/itm/KastKing-Spartacus-Low-Profile-Baitcaster-Baitcasting-Fishing-Reel/292121258171?var=591078728667

 

https://www.ebay.com/itm/KastKing-Spartacus-Plus-Baitcasting-Reel-Freshwater-Fishing-Rubber-Cork-Handle/183230403737?hash=item2aa9621499:m:mD7WmXAvNHXUBuTTePAM8Vg

 

https://www.dickssportinggoods.com/p/okuma-stratus-v-baitcasting-reel-15okuustrtsvlhbtcree/15okuustrtsvlhbtcree?