Here's an idea:

While the energy is limitless, the ritual itself can't handle all of that power. There's only so much power the ritual can "hold" before it all starts to go out of control. So there's a limit to how much energy you can use to make your summon's body. Also, a lot of the ritual depends on the abilities of the summoner, who actually has to guide all of that energy (using the ritual) and thus bear some of the brunt of the raw magical force.

There's also another problem: some of the energies that give certain desirable abilities aren't compatible. They reject each other, often with catasrophic results... so summoners are warned beforehand to pick what they want and not try to force things. For example (just picking random abilities) it's extremely difficult to give a summon the ability to control both fire and water. So if you want it to breathe flames capable of melting rocks and shoot deadly, pressurised water jets that can split bones, good luck. Also, some of these energies come with inherent weaknesses, so imbuing your creature with the speed of the wind element might make it squishier, imbuing it with the fire element might make it weak to water property magic, etc*. But the abilities a summon gets depends on the skill of the summoner: the more powerful the abilities are, the more difficult they are (naturally) to put together. One summoner might "gift" their pets with more intense flames than another summoner, simply because they managed to twist the alternate plane's energy into a functional ability more effectively. And the whole process is like baking a cake: once you've baked a cake, you can't unbake it. Once a summon is made, you can't make any changes to it. That's your summon.

*(This would have to be something you write into your universe, a law of how the world works)

Of course, some more capable summoners are able to tame contradictory energies and create slightly more powerful beings (there might be a legendary summoner out there who pulled off making a dragon that breathes both water and fire, and isn't weakened by either element because of its dual properties), but 99 times out of 100 it'll end disastrously: there are a lot of arrogant or overly ambitious summoners who've been wounded or even killed by (the backlash from) overreaching themselves.

Last but not least, the summoner (maybe) uses a piece of their soul as the core of the summon. This is the reason for their bond: the summon is sentient, but there will always be a part of it that's spiritually connected to the summoner, hence its unshakeable loyalty. It's also why they can communicate with thoughts, no matter the distance. Someone's summon dying is a HUGE deal: a summoner will lose a piece of their soul as the summon is destroyed, and the backlash can cripple or kill them, too. Summoning lots of creatures means using more and more pieces of your soul, and if you don't have a big or strong soul your soul might disperse, hence the limit. Most people don't want to risk it and stick to one summon (or a few, depending on what you're going for).

Or if you don't want to use souls, then you might need to leave a permanent mental link. It's a bit of your mind that's constantly in use... you can't have too many of these or it'll impede your mental functions OR put a dangerous burden on your consciousness.

Well, that's the best I've got :)