The number of blooms increases as healthy plants mature. If you plan to keep the plant in a container, it would prefer that the container was outside in full sun whenever temperatures allow (above freezing). The size/age of the plant and how much energy it has to devote to reproduction (blooms) ultimately determines bloom profusion. Since any energy the plant makes comes from sunshine, you should be giving your plant LOTS of it - and not through a window. Also, root maintenance, if the plant is to be left in the pot, is essential. You should let the plant go dormant over winter, lift it from the pot in spring, bare root/root prune it, and put it back in a/the pot to continue growing. If you don't maintain the roots, you can count on a plant limited in growth/vitality/bloom numbers. The plant doesn't tolerate wet feet, so count on using a very fast draining soil. The gritty mix would be perfect for this plant, and would make repotting chores easy. A 3:1:2 ratio fertilizer (examples: 24-8-16, 12-4-8, 9-3-6) used regularly is a very good choice. Al

Lavender is a real bug-a-boo. My French one looks worse than that. The Spanish one is looking good but it probably won't make it through winter. The English ones croaked. I decided to try them all to see if any would make it. They went downhill when we got humid in June (except the Spanish). Have you been humid there? That was my best guess here because they were planted in the ground in gritty soil, perfect drainage, full sun and I didn't water them since I read not to do that. I keep saying I will not try them again, then I do.

I would want to grow it as 'hard' as possible over the winter ie as cool as it will take and with plenty of light. That sappy anaemic leggy growth is going to be prey to any fungi and bugs going around and the plant really ought to be resting now, not trying to grow. I realise you can't put it outdoors but try to find a light cool place for it to tick over gently until spring. Then keep it outside as long as possible before it comes back in for the winter. Lavender is not well suited to an indoor life.

As balloonflower says, Lavender is a shrub and will naturally go woody at the base. I'd get them out of those plastic pots asap. As you do it you can rub off any loose brown leaves and that will let more air into the base of the plants. They look to me as if they have been kept rather crowded, a little lacking in light and probably pushed on with fertilisers to get a larger plant for sale. The kinked and pale stems show that. They need to be grown airy and a little lean to toughen them up.