It's great that you are taking into consideration the feedback from your survey respondents and looking for ways to improve the survey design.

Regarding the number of options that can be chosen in a multiple selection question, there is no set formula or evidence-based guideline on how many options to provide. The number of options to choose can depend on several factors, such as the goal of the survey, the target population, the complexity of the options, and the length of the survey.

In your case, where you are looking to gather information about the services provided by clinicians, it's important to keep in mind that you don't want to overload respondents with too many options, as this could lead to decreased response quality and accuracy. On the other hand, you also don't want to limit respondents too much and not allow them to fully express their views.

In this case, a good approach might be to provide a reasonable number of options (for example, 5-6), but also include an "other" option or a follow-up open-ended question where respondents can provide additional information. This can help to balance the need for specificity in the data while also allowing respondents to fully express their views.

In terms of research in this field, you can look into the literature on survey design, measurement, and psychometrics, as well as studies that have investigated the effects of different question formats and response options on response quality and accuracy. Some authors in this field include Darrell Huff, Douglas B. Fricker, and Donald R. Dillman.

One approach would be to allow as many responses as a respondent would like to put, but have them rank their choices. This way, you would not only discover all the choices that are being used, but the importance of those based on ranks. The dataset for each possible choice would be not selected or the rank that was selected. Good luck!

There is no clear-cut formula or rule when it comes to determining the number of options respondents should choose in multiple selection questions. It ultimately depends on the research question, the targeted population, and the nature of the options being provided. Some studies may require respondents to choose only one option, while others may allow them to select multiple options.

However, there are some best practices that researchers can follow when designing multiple selection questions. For example, researchers should ensure that the options provided are mutually exclusive and collectively exhaustive. This means that each option should represent a unique and distinct category, and that all possible categories are included in the options.

In terms of limiting the number of options respondents can choose, researchers can consider providing a clear rationale for why only a certain number of options can be selected. This can be explained in the survey instructions or in a pre-survey communication with respondents.

Overall, the key is to design questions that are clear, concise, and relevant to the research question at hand, while also taking into consideration the needs and preferences of the targeted population.