I have a 7 year old mare who struggles to keep the amount of weight I would like on her. She gets ridden 4-5 days a week and runs about 3 times a month of average. Also a very picky eater. Would love to hear some suggestions. 

Horses are meant to process fiber and nutrients via grass or legumes. The higher quality the hay the better your horse will gain and keep weight. Fat and fiber puts weight on. Since grasses and legumes don't have much in the way of fats you should look into a very high fat bagged supplement or feed. I chose to go with western alfalfa and renew gold mixed into beet pulp shreds (for some volume). There are other options of course, but the high quality hay goes further in putting on and holding weight than any bagged feed. Renew Gold is much more of a fat supplement than a feed (imo). I like it because it has fats from many different sources, rice bran, flax, coconut. I used to add in all those separately so I was very happy to switch over to one product. When I have one that needs some weight I will add in sunflower hearts and chips and vegetable oil also. Also, my horses are not stalled. I'm not sure how my get-fat-fast diet would work on a stalled horse. It might be too much energy.

Whats her diet you have her on now? Plus supplements. When was the last time her teeth were done and dewormed?

all mine eat is alfafa cubes morning and evening - bucket full

in the evening i add Purina Amplify 1lb bc it is 30% fat 

they have free choice hay in slow feed bags and mineral blocks

the sorrel is my 4yo jesses topaz and i bought her as a long yearling as she was in pitiful condition

the palomino is my 6yo by guys pocket coin and same she doesnt even look like the same horse as to when i bought her

I put mine on Purina Senior Active.   These are a couple of pictures of a 2yo I bought sight unseen.  Needless to say I was a little disappointed when I got her home in her condition.  But she filled out very well!  These photos are 2 weeks apart.   I actually looked and the Senior Active was very close to the Purina Ultium Growth formula.  Since my old horses were already on the SR, I just started feeding it to her.  

Get a bag of "Speedibeet". Luckily my feedstore carries it but its available online. 1-2 cups soaked a day and you will have a fatty on your hands! LOL Horses love it and lick their buckets way after its gone.. ANd its only 5% NSC so its great for preventing laminitis etc. 

She is currently eating as much alfalfa as she wants, 6lb Bluebonnet intesify omega force, along with MVP exceed 6-way. She is up to date on wormer and had her teeth done only a couple months ago. 

You might have already done this but I'd suggest treating for ulcers with gastro guard or ulcer gard . I feel your pain bc I have one exactly like you described ! I'm treating her for ulcers now & she's a different horse . Goes from picky & not eating to eating anything & everything . It's so frustrating when they won't gain weight when you are doing everything you can ! 

Five pounds total?  She needs more feed.  Or add rice bran. 

You have a few options depending on what you can get her to eat. I've been dealing with some hard keeping picky eaters lately. My go-to's are rice bran, beet pulp, or canola oil. Or a combination of those. If you're open to switching feeds, Triple Crown senior is incredibly good at putting weight on them.

Cool Calories. It's so fattening I use gloves to handle it. 

How many pounds would you suggest?

From what I've seen. 

Most people over estimate the nutritional value of the forage they are feeding. 

Purina has several products that include their Gastro Guard. My gelding really made a gain when I switched him to Strategy Healthy Edge (with Gastro Guard). It is very important to weigh your feed and feed per the directions on the feed bag based on your horse's activity level, and weight. 

Along with treating for ulcers I would also try feeding calf manna. That stuff usually puts the weight right on one. 

Is this 6 lbs in one feeding as in twice a day are do you split it into two feedings, as in 3 lbs in the morning and 3 lbs in the evening?

How much calf manna?