You are free to submit as many proposals as you like, but please make sure to accept invitations and offers for jobs you're sure you have time and skill to complete.
Before I write a complete comment related to the OP, please understand I only work long term jobs. So while my answer might not apply to those who do short term jobs, it might be a possible tip for those who are usually engaged in long term gigs. My gigs typically last 6 months to a year. And for personal reasons, I prefer not to work more then 35 hours a week, on average.
1. I typically draw the line at 2 active clients, as most of my clients need me for at least 8-15 hours a week, or more. If one of these clients becomes inactive for a while, I will change my "not available until (date)" time to "less then 30 hours".
2. If I have only one client, and feel like they don't need more more then 20 hours a week, I might persue another client, on the premise that they would not need me more then 15 hours a week.
3. If I am fielding a lot of proposals and interviews (I only do this when I have an open client slot, out of the 2 that I usually have), I will often get get hired before prospective clients have a chance to make a decision. If this happens, I will message all the other clients who I already interviewed with and let them know that I can't work for them NOW, but could in a few months.
4. If I get a private invite with a really good opportunity (rare but it happens) when I have no hours available, I will message the client and let them know I'm interested, but can't work for them right now.